#1290: recent libraries are checked on workers, never on the UI thread

Review round (CodeRabbit + an adversarial review, handled together):
- The launch list starts with every row "Checking…" and a worker per path checks it (a
  share whose server is gone, or a sleeping disk, can take seconds or never answer; one
  that hangs doesn't hold the others). After 5 s a row says the drive isn't answering;
  it can still be opened. Once every answer is in, the selection moves to the first
  library that can be opened, unless the person moved it.
- File ▸ Open Recent Library and `app.recentLibraries` no longer look at folders either
  (the menu is rebuilt every frame): they use the same answers, refreshed after 30 s, when
  a library is opened, removed from the list, or fails to open. `app.recentLibraries`
  gives `exists: null` and `checking` until a check answered.
- Reasons are found on the worker and put into words on the UI thread, in the interface
  language. A check whose worker dies ends as "This library couldn't be checked".
- Each check holds an activity row ("Checking recent libraries"; quick ones don't show).
- Docs: control protocol (`checking`, `available`, `exists`), What's New; fr strings.
Tests: slow check never holds the window; late answers keep the person's selection; the
menu takes its answers from a worker (fails with the old is_dir()); reasons in French; one
hung drive doesn't hold the others; a dying check ends; reopening clears a missing answer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/lightcraft/src/main.rs

@@ -980,9 +980,9 @@ fn main() -> eframe::Result {
                 if let Some(ui) = prefs {
                     app.ui = ui;
                 }
-                // the recent libraries to choose from (issue #1290), each checked now
+                // the recent libraries to choose from (issue #1290), checked on a worker once shown
                 if launch == Launch::Ask {
-                    problem = Some(LibraryProblem::choose(&app.ui.settings));
+                    problem = Some(LibraryProblem::choose_later(&app.ui.settings));
                 }
                 app.alt_asks_for_library = matches!(launch, Launch::Open { alt_asks: true, .. });
                 lightcraft_ui_egui::i18n::set_language(app.ui.language);
@@ -1177,7 +1177,7 @@ mod tests {
         let path = d.join("ui.json");
         let mut app = LightcraftApp::new(s, Services::default());
         app.ui.settings.recent_libraries = vec!["/gone/a".into(), "/gone/b".into()];
-        app.library_problem = Some(LibraryProblem::choose(&app.ui.settings));
+        app.library_problem = Some(LibraryProblem::choose_later(&app.ui.settings));
         let mut w = PrefsWriter { path: Some(path.clone()), ..Default::default() };
         app.ui.settings.recent_libraries.remove(0);
         w.save(&app).unwrap();

crates/ui-egui/locales/fr.json

@@ -2358,5 +2358,8 @@
   "There is no LightCraft library in this folder any more": "Ce dossier ne contient plus de bibliothèque LightCraft",
   "This is a file, not a library folder": "C’est un fichier, pas un dossier de bibliothèque",
   "This folder can't be read:": "Ce dossier ne peut pas être lu :",
-  "The library's catalog can't be read:": "Le catalogue de la bibliothèque ne peut pas être lu :"
+  "The library's catalog can't be read:": "Le catalogue de la bibliothèque ne peut pas être lu :",
+  "Not answering yet: the drive may be asleep or offline": "Pas encore de réponse : le disque est peut-être en veille ou hors ligne",
+  "This library couldn't be checked": "Impossible de vérifier cette bibliothèque",
+  "Checking recent libraries": "Vérification des bibliothèques récentes"
 }

crates/ui-egui/src/lib.rs

@@ -488,6 +488,9 @@ pub struct LightcraftApp {
     /// Alt/Option held as the window first appears offers the recent libraries once (issue
     /// #1290); the desktop host arms it when nothing chose the library already.
     pub alt_asks_for_library: bool,
+    /// Whether each recent library can be opened, answered by a worker (issue #1290): the launch
+    /// list and File ▸ Open Recent Library never look at a folder on the UI thread.
+    pub recent_checks: panels::library_problem::RecentChecks,
     /// The activity stack shows every task, not just the first few ("+N more" was clicked).
     pub activity_expanded: bool,
 }
@@ -568,6 +571,7 @@ impl LightcraftApp {
             screen_profile_read: None,
             library_problem: None,
             alt_asks_for_library: false,
+            recent_checks: Default::default(),
             activity_expanded: false,
             model_setup: Default::default(),
         }
@@ -1431,6 +1435,8 @@ impl LightcraftApp {
         if let Some(l) = self.session.library.as_ref().filter(|l| l.on_disk) {
             let path = l.dir.to_string_lossy().into_owned();
             panels::settings::remember_library(&mut self.ui.settings.recent_libraries, &path);
+            // it is open: whatever an earlier check said about it is over
+            self.recent_checks.opened(&path);
             // when, for the launch window (issue #1290); only the listed libraries keep a time
             let settings = &mut self.ui.settings;
             settings.recent_opened.insert(path, (self.session.clock)());

crates/ui-egui/src/menubar.rs

@@ -641,8 +641,10 @@ pub(crate) fn expanded(app: &LightcraftApp, name: &str) -> Option<Vec<MenuNode>>
                 .iter()
                 .map(|r| {
                     let d = std::path::Path::new(r);
-                    // a missing library (moved, drive disconnected) is shown greyed out
-                    item("app.openLibrary", json!({"path": r}), label(r), None, can && d.is_dir(), Some(open.as_deref() == Some(d)))
+                    // a library a check found missing (moved, drive disconnected) is greyed out; the
+                    // check runs on a worker (`RecentChecks`), never here: the menu is rebuilt every frame
+                    let gone = app.recent_checks.reason(r).is_some();
+                    item("app.openLibrary", json!({"path": r}), label(r), None, can && !gone, Some(open.as_deref() == Some(d)))
                 })
                 .collect();
             if v.is_empty() {

crates/ui-egui/src/panels/library_problem.rs

@@ -7,6 +7,12 @@
 //! is *Ask me*, or when Alt/Option is held as the window first appears (issue #1290): Open, Open
 //! Another…, New Library…, Remove from List and Quit, with the arrow keys, Return and Esc.
 //! [`launch_plan`] decides at launch which of the two the desktop app starts with.
+//!
+//! Whether each recent library can be opened is checked on a worker ([`RecentChecks`]), never on
+//! the UI thread: a library on a network share whose server is gone, or on a disk that has to
+//! spin up, can take seconds to answer. Until its answer comes the list shows the row as
+//! "Checking…" (it can still be opened: opening says why if it can't), and File ▸ Open Recent
+//! Library keeps it enabled; the same answers grey out the menu's entries.
 
 use std::path::{Path, PathBuf};
 
@@ -56,9 +62,13 @@ pub struct RecentLibrary {
     pub path: String,
     /// When it was last opened (ISO 8601), if known.
     pub opened: Option<String>,
-    /// Why it can't be opened (`None`: it can), as checked when the window opened or when
-    /// opening it failed.
+    /// Why it can't be opened (`None`: it can, or it is still being checked), as checked when
+    /// the window opened or when opening it failed.
     pub unavailable: Option<String>,
+    /// Its check hasn't answered yet ([`RecentChecks`]).
+    pub checking: bool,
+    /// Its check has gone on for longer than [`SLOW_AFTER`] (the drive may be asleep or offline).
+    pub slow: bool,
 }
 
 /// The launch window's list of recent libraries (issue #1290).
@@ -68,6 +78,10 @@ pub struct Chooser {
     pub libraries: Vec<RecentLibrary>,
     /// The highlighted row (`None`: the list is empty).
     pub selected: Option<usize>,
+    /// The person moved the highlight: answers arriving later don't move it again.
+    pub touched: bool,
+    /// Its rows' checks were started ([`logic`]).
+    pub asked: bool,
     /// The library open behind the window (Alt/Option held at launch), by
     /// [`lightcraft_engine::Session::library_generation`]: Esc keeps it. `None`: no library is open.
     pub over: Option<u64>,
@@ -102,21 +116,40 @@ impl LibraryProblem {
     /// ([`unavailable`]): one that is gone, isn't a LightCraft library any more or can't be read
     /// is greyed out with the reason. The last library opened is selected, or else the first one
     /// that can be opened.
-    pub fn choose(settings: &AppSettings) -> LibraryProblem {
+    #[cfg(test)]
+    pub(crate) fn choose(settings: &AppSettings) -> LibraryProblem {
+        let mut p = LibraryProblem::choose_later(settings);
+        if let Some(c) = p.chooser.as_mut() {
+            for l in &mut c.libraries {
+                l.unavailable = unavailable(&l.path).map(|u| u.text());
+                l.checking = false;
+            }
+            c.selected = initial_selection(&c.libraries, &settings.library_path);
+            c.asked = true;
+        }
+        p
+    }
+
+    /// [`LibraryProblem::choose`] without looking at any folder: every row is "Checking…" until
+    /// [`logic`] has the answers from a worker ([`RecentChecks`]). The last library opened is
+    /// selected meanwhile (or the first row); once every answer is in, the selection follows the
+    /// same rule as [`LibraryProblem::choose`] unless the person moved it.
+    pub fn choose_later(settings: &AppSettings) -> LibraryProblem {
         // as many as File ▸ Open Recent Library keeps (a hand-edited settings file may hold more)
         let libraries: Vec<RecentLibrary> = settings
             .recent_libraries
             .iter()
             .take(crate::state::RECENT_LIBRARIES)
-            .map(|path| RecentLibrary { path: path.clone(), opened: settings.recent_opened.get(path).cloned(), unavailable: unavailable(path) })
+            .map(|path| RecentLibrary {
+                path: path.clone(),
+                opened: settings.recent_opened.get(path).cloned(),
+                unavailable: None,
+                checking: true,
+                slow: false,
+            })
             .collect();
-        let can = |l: &RecentLibrary| l.unavailable.is_none();
-        let selected = libraries
-            .iter()
-            .position(|l| l.path == settings.library_path && can(l))
-            .or_else(|| libraries.iter().position(can))
-            .or((!libraries.is_empty()).then_some(0));
-        LibraryProblem { chooser: Some(Chooser { libraries, selected, over: None }), ..Default::default() }
+        let selected = libraries.iter().position(|l| l.path == settings.library_path).or((!libraries.is_empty()).then_some(0));
+        LibraryProblem { chooser: Some(Chooser { libraries, selected, over: None, touched: false, asked: false }), ..Default::default() }
     }
 
     /// The session promises to save nothing (the failed-library window or Continue Without
@@ -131,7 +164,7 @@ impl LibraryProblem {
             let libraries: Vec<serde_json::Value> = c
                 .libraries
                 .iter()
-                .map(|l| json!({"path": l.path, "lastOpened": l.opened, "available": l.unavailable.is_none(), "reason": l.unavailable}))
+                .map(|l| json!({"path": l.path, "lastOpened": l.opened, "available": l.unavailable.is_none(), "checking": l.checking, "reason": l.unavailable}))
                 .collect();
             json!({"libraries": libraries, "selected": c.selected, "libraryOpen": c.over.is_some()})
         });
@@ -139,32 +172,220 @@ impl LibraryProblem {
     }
 }
 
+/// The row first selected: the last library opened if it can be opened, else the first one that
+/// can, else the first row.
+fn initial_selection(libraries: &[RecentLibrary], last: &str) -> Option<usize> {
+    let can = |l: &RecentLibrary| l.unavailable.is_none();
+    libraries.iter().position(|l| l.path == last && can(l)).or_else(|| libraries.iter().position(can)).or((!libraries.is_empty()).then_some(0))
+}
+
+/// Why a recent library can't be opened, found on a worker and put into words on the UI thread
+/// (the interface language is per thread).
+#[derive(Clone, Debug, PartialEq)]
+pub enum Unavailable {
+    /// Nothing at that path (moved, renamed, or on a disconnected drive).
+    NotFound,
+    /// A file, not a folder.
+    NotAFolder,
+    /// The folder can't be listed (the system's reason).
+    Unreadable(String),
+    /// The folder holds no LightCraft library any more.
+    NoLibrary,
+    /// The library's catalog file can't be opened (the system's reason).
+    CatalogUnreadable(String),
+    /// The check itself failed (its worker stopped, or couldn't start).
+    CheckFailed(String),
+}
+
+impl Unavailable {
+    /// In the interface language.
+    pub fn text(&self) -> String {
+        match self {
+            Unavailable::NotFound => crate::i18n::tr("Not found: moved, renamed or on a disconnected drive").to_string(),
+            Unavailable::NotAFolder => crate::i18n::tr("This is a file, not a library folder").to_string(),
+            Unavailable::Unreadable(e) => format!("{} {e}", crate::i18n::tr("This folder can't be read:")),
+            Unavailable::NoLibrary => crate::i18n::tr("There is no LightCraft library in this folder any more").to_string(),
+            Unavailable::CatalogUnreadable(e) => format!("{} {e}", crate::i18n::tr("The library's catalog can't be read:")),
+            Unavailable::CheckFailed(e) => e.clone(),
+        }
+    }
+}
+
+/// How long an answer stands before File ▸ Open Recent Library asks again (a drive plugged back
+/// in, a library moved away meanwhile).
+pub const RECHECK_AFTER: std::time::Duration = std::time::Duration::from_secs(30);
+/// How long a row stays "Checking…" before it says the drive isn't answering (it can still be
+/// opened: opening says why if it can't).
+pub const SLOW_AFTER: std::time::Duration = std::time::Duration::from_secs(5);
+
+/// The activity stack's row for a check (English: the stack translates it).
+const CHECK_LABEL: &str = "Checking recent libraries";
+
+/// One path's answer and when it came.
+#[derive(Clone, Debug)]
+struct Answer {
+    why: Option<Unavailable>,
+    at: web_time::Instant,
+}
+
+/// Answers to "can this recent library be opened?", each found on a worker thread of its own (a
+/// library on a network share whose server is gone, or on a sleeping disk, can take seconds or
+/// never answer; the others don't wait for it).
+pub struct RecentChecks {
+    known: std::collections::HashMap<String, Answer>,
+    /// Paths being checked, and since when.
+    pending: std::collections::HashMap<String, web_time::Instant>,
+    /// How one path is checked ([`unavailable`]; tests put a slow one here).
+    check: std::sync::Arc<dyn Fn(&str) -> Option<Unavailable> + Send + Sync>,
+}
+
+impl Default for RecentChecks {
+    /// Nothing known yet, checking with [`unavailable`].
+    fn default() -> Self {
+        RecentChecks { known: Default::default(), pending: Default::default(), check: std::sync::Arc::new(unavailable) }
+    }
+}
+
+impl RecentChecks {
+    /// Why `path` can't be opened, if a check said so (`None`: it can, or it isn't known yet).
+    pub fn reason(&self, path: &str) -> Option<&Unavailable> {
+        self.known.get(path).and_then(|a| a.why.as_ref())
+    }
+
+    /// Whether a check of `path` has answered.
+    pub fn is_known(&self, path: &str) -> bool {
+        self.known.contains_key(path)
+    }
+
+    /// Whether `path` is being checked.
+    pub fn is_checking(&self, path: &str) -> bool {
+        self.pending.contains_key(path)
+    }
+
+    /// Since when `path` has been checked, if it is.
+    fn checking_since(&self, path: &str) -> Option<web_time::Instant> {
+        self.pending.get(path).copied()
+    }
+
+    /// A check is running.
+    pub fn is_running(&self) -> bool {
+        !self.pending.is_empty()
+    }
+
+    /// Check paths with another function (tests).
+    pub fn with_check(check: impl Fn(&str) -> Option<Unavailable> + Send + Sync + 'static) -> RecentChecks {
+        RecentChecks { check: std::sync::Arc::new(check), ..Default::default() }
+    }
+
+    /// `path` was just opened (or made): it can be opened.
+    pub fn opened(&mut self, path: &str) {
+        self.known.insert(path.to_string(), Answer { why: None, at: web_time::Instant::now() });
+    }
+
+    /// Opening `path` failed: what the open said is its answer until the next check.
+    pub fn failed(&mut self, path: &str, error: &str) {
+        self.known.insert(path.to_string(), Answer { why: Some(Unavailable::CheckFailed(error.to_string())), at: web_time::Instant::now() });
+    }
+
+    /// `path` left the list: forget its answer.
+    pub fn forget(&mut self, path: &str) {
+        self.known.remove(path);
+    }
+}
+
+/// Check each of `paths` on a worker of its own (one that is being checked already is left to
+/// finish). The activity stack lists each check while it runs (a quick one doesn't show: rows
+/// appear after half a second); none can be stopped, as a stat that hangs can't be.
+fn start_checks(app: &mut LightcraftApp, paths: Vec<String>) {
+    for path in paths {
+        if app.recent_checks.pending.contains_key(&path) {
+            continue;
+        }
+        let check = app.recent_checks.check.clone();
+        let p = path.clone();
+        let started = crate::tasks::spawn(app, CHECK_LABEL, Some("library"), move || check(&p), {
+            let path = path.clone();
+            move |app, ctx, why| {
+                app.recent_checks.pending.remove(&path);
+                app.recent_checks.known.insert(path, Answer { why, at: web_time::Instant::now() });
+                apply_answers(app);
+                ctx.request_repaint();
+            }
+        });
+        match started {
+            Ok(()) => {
+                app.recent_checks.pending.insert(path, web_time::Instant::now());
+            }
+            // no worker thread: say so on the row, and don't try again every frame
+            Err(e) => {
+                log::warn!("{e}");
+                app.recent_checks.known.insert(path, Answer { why: Some(Unavailable::CheckFailed(e)), at: web_time::Instant::now() });
+                apply_answers(app);
+            }
+        }
+    }
+}
+
+/// Checks whose worker stopped without answering (it panicked: `tasks::poll` already said so)
+/// end as failed, so the row and the menu don't wait for them forever.
+fn reap_checks(app: &mut LightcraftApp) {
+    // (every check has the same label: when none is listed any more, the ones still pending died)
+    if app.recent_checks.pending.is_empty() || app.tasks.is_running(CHECK_LABEL) {
+        return;
+    }
+    let dead: Vec<String> = app.recent_checks.pending.keys().cloned().collect();
+    for path in dead {
+        app.recent_checks.pending.remove(&path);
+        let why = Unavailable::CheckFailed(crate::i18n::tr("This library couldn't be checked").to_string());
+        app.recent_checks.known.insert(path, Answer { why: Some(why), at: web_time::Instant::now() });
+    }
+    apply_answers(app);
+}
+
+/// Put the answers in into the launch list; once none is missing, select as
+/// [`LibraryProblem::choose`] does unless the person moved the highlight.
+fn apply_answers(app: &mut LightcraftApp) {
+    let last = app.ui.settings.library_path.clone();
+    let checks = &app.recent_checks;
+    let Some(c) = app.library_problem.as_mut().and_then(|p| p.chooser.as_mut()) else { return };
+    let checking_before = c.libraries.iter().any(|l| l.checking);
+    for l in &mut c.libraries {
+        if l.checking && !checks.is_checking(&l.path) && checks.is_known(&l.path) {
+            l.unavailable = checks.reason(&l.path).map(Unavailable::text);
+            l.checking = false;
+        }
+    }
+    if checking_before && !c.libraries.iter().any(|l| l.checking) && !c.touched {
+        c.selected = initial_selection(&c.libraries, &last);
+    }
+}
+
 /// Why the library at `path` can't be opened (`None`: as far as can be told without opening it).
 ///
 /// Read-only: the folder and its catalog file are looked at, never locked or written (the list
 /// may name libraries on other drives, network shares or sleeping disks). A library another
 /// program has open is therefore found when Open is pressed: the open fails and the window says
-/// who has it.
-fn unavailable(path: &str) -> Option<String> {
+/// who has it. Runs on a worker: the reason is put into words on the UI thread.
+fn unavailable(path: &str) -> Option<Unavailable> {
     let dir = Path::new(path);
     if !dir.exists() {
-        return Some(crate::i18n::tr("Not found: moved, renamed or on a disconnected drive").to_string());
+        return Some(Unavailable::NotFound);
     }
     if !dir.is_dir() {
-        return Some(crate::i18n::tr("This is a file, not a library folder").to_string());
+        return Some(Unavailable::NotAFolder);
     }
     if let Err(e) = std::fs::read_dir(dir) {
-        return Some(format!("{} {e}", crate::i18n::tr("This folder can't be read:")));
+        return Some(Unavailable::Unreadable(e.to_string()));
     }
     // opening would make a new, empty library in it
     if !lightcraft_engine::library::is_library(dir) {
-        return Some(crate::i18n::tr("There is no LightCraft library in this folder any more").to_string());
+        return Some(Unavailable::NoLibrary);
     }
     // the catalog the open reads first: the snapshot, else the log alone (opened to read, not read)
     let snapshot = dir.join(lightcraft_catalog::journal::SNAPSHOT);
     let catalog = if snapshot.is_file() { snapshot } else { dir.join(lightcraft_catalog::journal::LOG) };
     if let Err(e) = std::fs::File::open(&catalog) {
-        return Some(format!("{} {e}", crate::i18n::tr("The library's catalog can't be read:")));
+        return Some(Unavailable::CatalogUnreadable(e.to_string()));
     }
     None
 }
@@ -207,6 +428,9 @@ fn open(app: &mut LightcraftApp, path: Option<String>) {
         Err(e) => {
             // a failed switch reopens the library that was open behind the launch list
             let generation = app.session.library.as_ref().map(|_| app.session.library_generation());
+            if let Some(path) = path.as_deref() {
+                app.recent_checks.failed(path, &e);
+            }
             if let Some(p) = app.library_problem.as_mut() {
                 if let Some(c) = p.chooser.as_mut() {
                     if c.over.is_some() {
@@ -214,6 +438,7 @@ fn open(app: &mut LightcraftApp, path: Option<String>) {
                     }
                     if let Some(l) = c.libraries.iter_mut().find(|l| path.as_deref() == Some(l.path.as_str())) {
                         l.unavailable = Some(e.clone());
+                        l.checking = false;
                     }
                 }
                 if let Some(path) = path {
@@ -244,6 +469,41 @@ pub fn logic(app: &mut LightcraftApp, ctx: &egui::Context) {
         }
     }
     alt_at_launch(app, ctx);
+    reap_checks(app);
+    // the launch list asks for fresh answers once
+    let listed: Option<Vec<String>> =
+        app.library_problem.as_mut().and_then(|p| p.chooser.as_mut()).and_then(|c| {
+            (!std::mem::replace(&mut c.asked, true)).then(|| c.libraries.iter().filter(|l| l.checking).map(|l| l.path.clone()).collect())
+        });
+    if let Some(listed) = listed {
+        for path in &listed {
+            if !app.recent_checks.is_checking(path) {
+                app.recent_checks.forget(path);
+            }
+        }
+        start_checks(app, listed);
+        apply_answers(app);
+    }
+    // File ▸ Open Recent Library: paths never checked, or not for a while
+    let stale: Vec<String> = app
+        .ui
+        .settings
+        .recent_libraries
+        .iter()
+        .filter(|p| !app.recent_checks.is_checking(p) && app.recent_checks.known.get(*p).is_none_or(|a| a.at.elapsed() > RECHECK_AFTER))
+        .cloned()
+        .collect();
+    start_checks(app, stale);
+    if app.recent_checks.is_running() {
+        // rows still checking say so once it takes long
+        let checks = &app.recent_checks;
+        if let Some(c) = app.library_problem.as_mut().and_then(|p| p.chooser.as_mut()) {
+            for l in c.libraries.iter_mut().filter(|l| l.checking) {
+                l.slow = checks.checking_since(&l.path).is_some_and(|t| t.elapsed() > SLOW_AFTER);
+            }
+        }
+        ctx.request_repaint_after(std::time::Duration::from_millis(250));
+    }
     chooser_keys(app, ctx);
 }
 
@@ -266,7 +526,7 @@ fn alt_at_launch(app: &mut LightcraftApp, ctx: &egui::Context) {
     if crate::panels::settings::can_switch_library(app).is_err() {
         return;
     }
-    let mut p = LibraryProblem::choose(&app.ui.settings);
+    let mut p = LibraryProblem::choose_later(&app.ui.settings);
     if let Some(c) = p.chooser.as_mut() {
         c.over = app.session.library.as_ref().map(|_| app.session.library_generation());
     }
@@ -314,6 +574,7 @@ fn select(app: &mut LightcraftApp, at: usize) {
         && at < c.libraries.len()
     {
         c.selected = Some(at);
+        c.touched = true;
     }
 }
 
@@ -410,6 +671,7 @@ fn act(app: &mut LightcraftApp, choice: Choice) {
         Choice::Remove => {
             let Some((library, _)) = chosen else { return };
             crate::panels::settings::forget_library(&mut app.ui.settings, &library.path);
+            app.recent_checks.forget(&library.path);
             if let Some(c) = app.library_problem.as_mut().and_then(|p| p.chooser.as_mut()) {
                 c.libraries.retain(|l| l.path != library.path);
                 c.selected = c.selected.map(|s| s.min(c.libraries.len().saturating_sub(1))).filter(|_| !c.libraries.is_empty());
@@ -549,6 +811,10 @@ fn recent_row(ui: &mut egui::Ui, t: &Tokens, i: usize, library: &RecentLibrary,
         ui.add(egui::Label::new(RichText::new(parent).size(11.0).color(dim)).truncate().selectable(false));
         if let Some(why) = &library.unavailable {
             ui.add(egui::Label::new(RichText::new(why).size(11.0).color(t.caution)).wrap().selectable(false));
+        } else if library.checking {
+            let text =
+                if library.slow { crate::i18n::tr("Not answering yet: the drive may be asleep or offline") } else { crate::i18n::tr("Checking…") };
+            ui.add(egui::Label::new(RichText::new(text).size(11.0).color(dim)).wrap().selectable(false));
         }
     });
     let r = ui.interact(inner.response.rect, egui::Id::new(("library-recent", i)), egui::Sense::click());

crates/ui-egui/src/panels/settings.rs

@@ -1306,8 +1306,11 @@ fn free_name(location: &std::path::Path, base: &str) -> std::path::PathBuf {
 }
 
 /// `app.recentLibraries {remove?: path, clear?: bool}`: the recently opened libraries, newest
-/// first, as `[{path, name, exists, open, lastOpened}]` (`exists: false`: moved or on a
-/// disconnected drive; `lastOpened`: ISO 8601 time, `null` when not known). `remove` forgets that
+/// first, as `[{path, name, exists, checking, open, lastOpened}]`. `exists` is what the last
+/// background check found (`false`: moved, on a disconnected drive or no longer a library;
+/// `null`: not checked yet, `checking` then says whether a check is running): the command never
+/// looks at a folder itself, as one on an offline network share can take seconds to answer.
+/// `lastOpened`: ISO 8601 time, `null` when not known. `remove` forgets that
 /// one entry (an error if the list doesn't hold it) and `clear` forgets them all; neither touches
 /// anything on disk (issue #1290).
 pub fn recent_libraries(app: &mut LightcraftApp, p: &Value) -> Result<Value, String> {
@@ -1317,21 +1320,25 @@ pub fn recent_libraries(app: &mut LightcraftApp, p: &Value) -> Result<Value, Str
             if !forget_library(&mut app.ui.settings, path) {
                 return Err(format!("app.recentLibraries: {path:?} is not in the recent libraries"));
             }
+            app.recent_checks.forget(path);
         }
         Some(_) => return Err("app.recentLibraries: remove must be a library path (a string)".into()),
     }
     if p.get("clear").and_then(Value::as_bool) == Some(true) {
-        app.ui.settings.recent_libraries.clear();
+        for path in std::mem::take(&mut app.ui.settings.recent_libraries) {
+            app.recent_checks.forget(&path);
+        }
         app.ui.settings.recent_opened.clear();
     }
     let open = app.session.library.as_ref().filter(|l| l.on_disk).map(|l| l.dir.clone());
-    let settings = &app.ui.settings;
+    let (settings, checks) = (&app.ui.settings, &app.recent_checks);
     let list: Vec<Value> = settings
         .recent_libraries
         .iter()
         .map(|r| {
             let d = std::path::Path::new(r);
-            json!({"path": r, "name": library_name(d), "exists": d.is_dir(), "open": open.as_deref() == Some(d), "lastOpened": settings.recent_opened.get(r)})
+            let exists = checks.is_known(r).then(|| checks.reason(r).is_none());
+            json!({"path": r, "name": library_name(d), "exists": exists, "checking": checks.is_checking(r), "open": open.as_deref() == Some(d), "lastOpened": settings.recent_opened.get(r)})
         })
         .collect();
     Ok(Value::Array(list))

crates/ui-egui/src/tests_launch_library.rs

@@ -417,7 +417,8 @@ fn open_recent_library_and_the_command_remove_one_entry() {
 
     // the command: the open library can be forgotten too (it stays open), and nothing is deleted
     let list = app.run("app.recentLibraries", json!({"remove": first})).unwrap();
-    assert_eq!(list, json!([{"path": "/elsewhere/third", "name": "third", "exists": false, "open": false, "lastOpened": null}]));
+    // (no frame ran: nothing checked yet, so `exists` is not known; the command never looks itself)
+    assert_eq!(list, json!([{"path": "/elsewhere/third", "name": "third", "exists": null, "checking": false, "open": false, "lastOpened": null}]));
     assert!(app.session.library.is_some() && lightcraft_engine::library::is_library(&dir.join("first")));
     let e = app.run("app.recentLibraries", json!({"remove": first})).unwrap_err();
     assert!(e.contains("not in the recent libraries"), "{e}");
@@ -574,3 +575,213 @@ fn alt_is_only_watched_while_the_window_first_appears_and_when_armed() {
     assert!(app.library_problem.as_ref().is_some_and(|p| p.chooser.is_some()));
     assert!(!app.alt_asks_for_library);
 }
+
+/// A check that waits until the test lets it go (a network share whose server is gone), then
+/// answers like the real one for a missing folder. `release` lets every waiting check go.
+fn slow_checks() -> (
+    crate::panels::library_problem::RecentChecks,
+    std::sync::Arc<(std::sync::Mutex<bool>, std::sync::Condvar)>,
+    std::sync::Arc<std::sync::atomic::AtomicUsize>,
+) {
+    let gate = std::sync::Arc::new((std::sync::Mutex::new(false), std::sync::Condvar::new()));
+    let calls = std::sync::Arc::new(std::sync::atomic::AtomicUsize::new(0));
+    let (g, n) = (gate.clone(), calls.clone());
+    let checks = crate::panels::library_problem::RecentChecks::with_check(move |p| {
+        n.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
+        let (lock, cv) = &*g;
+        let open = lock.lock().unwrap();
+        // (bounded: a test that forgets to release still ends)
+        let _unused = cv.wait_timeout_while(open, Duration::from_secs(10), |open| !*open).unwrap();
+        (!Path::new(p).exists()).then_some(crate::panels::library_problem::Unavailable::NotFound)
+    });
+    (checks, gate, calls)
+}
+
+/// Let every check waiting in [`slow_checks`] answer.
+fn release(gate: &std::sync::Arc<(std::sync::Mutex<bool>, std::sync::Condvar)>) {
+    let (lock, cv) = &**gate;
+    *lock.lock().unwrap() = true;
+    cv.notify_all();
+}
+
+/// Review of #2701: the launch list never waits on a folder. Its rows show "Checking…" while a
+/// worker looks (the activity stack lists it), the window keeps drawing frames meanwhile, and the
+/// answers grey out a missing library and move the selection to one that can be opened.
+#[test]
+fn a_slow_check_never_holds_the_launch_window() {
+    let dir = temp("slow");
+    let a = make_library(&dir.join("A"));
+    let gone = dir.join("Gone").to_string_lossy().into_owned();
+    let mut app = LightcraftApp::new(lightcraft_engine::Session::new().with_fs(), Services::default());
+    app.ui.settings.recent_libraries = vec![gone.clone(), a.clone()];
+    app.ui.settings.library_path = gone.clone();
+    let (checks, gate, _) = slow_checks();
+    app.recent_checks = checks;
+    app.library_problem = Some(LibraryProblem::choose_later(&app.ui.settings));
+    let mut h = Headless::new(app, [1300.0, 900.0], 1.0);
+    let t0 = std::time::Instant::now();
+    settle(&mut h);
+    assert!(t0.elapsed() < Duration::from_secs(5), "frames went on while the check waited: {:?}", t0.elapsed());
+    let c = h.app.library_problem.as_ref().and_then(|p| p.chooser.clone()).unwrap();
+    assert!(c.libraries.iter().all(|l| l.checking && l.unavailable.is_none()), "{c:?}");
+    assert_eq!(c.selected, Some(0), "the last library opened, until the answers say otherwise");
+    assert!(h.painted_text().iter().any(|t| t == "Checking…"), "{:?}", h.painted_text());
+    assert!(h.app.session.activity.list().iter().any(|t| t.label == "Checking recent libraries"), "the activity stack lists the check");
+
+    release(&gate);
+    let done =
+        h.step_until(T, |h| h.app.library_problem.as_ref().and_then(|p| p.chooser.as_ref()).is_some_and(|c| c.libraries.iter().all(|l| !l.checking)));
+    assert!(done, "the answers came back");
+    let c = h.app.library_problem.as_ref().and_then(|p| p.chooser.clone()).unwrap();
+    assert!(c.libraries[0].unavailable.as_deref().is_some_and(|w| w.contains("Not found")), "{c:?}");
+    assert_eq!(c.libraries[1].unavailable, None);
+    assert_eq!(c.selected, Some(1), "the first library that can be opened, once known");
+    assert!(h.app.session.activity.list().iter().all(|t| t.label != "Checking recent libraries"), "the row goes with the check");
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
+/// Answers that arrive after the person moved the highlight leave it where they put it.
+#[test]
+fn late_answers_keep_the_row_the_person_chose() {
+    let dir = temp("touched");
+    let a = make_library(&dir.join("A"));
+    let gone = dir.join("Gone").to_string_lossy().into_owned();
+    let mut app = LightcraftApp::new(lightcraft_engine::Session::new().with_fs(), Services::default());
+    app.ui.settings.recent_libraries = vec![a.clone(), gone.clone()];
+    app.ui.settings.library_path = a.clone();
+    let (checks, gate, _) = slow_checks();
+    app.recent_checks = checks;
+    app.library_problem = Some(LibraryProblem::choose_later(&app.ui.settings));
+    let mut h = Headless::new(app, [1300.0, 900.0], 1.0);
+    settle(&mut h);
+    h.request("ui.key", json!({"key": "ArrowDown"}), T);
+    settle(&mut h);
+    release(&gate);
+    h.step_until(T, |h| h.app.library_problem.as_ref().and_then(|p| p.chooser.as_ref()).is_some_and(|c| c.libraries.iter().all(|l| !l.checking)));
+    let c = h.app.library_problem.as_ref().and_then(|p| p.chooser.clone()).unwrap();
+    assert_eq!(c.selected, Some(1), "still the row moved to, though it can't be opened");
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
+/// File ▸ Open Recent Library is rebuilt every frame: it never looks at a folder itself. A
+/// library is greyed out only once a worker found it missing, and each path is checked once.
+#[test]
+fn the_recent_menu_takes_its_answers_from_a_worker() {
+    use crate::menubar::MenuNode;
+    let dir = temp("menu-worker");
+    let gone = dir.join("Gone").to_string_lossy().into_owned();
+    let mut app = LightcraftApp::new(lightcraft_engine::Session::new().with_fs(), Services::default());
+    app.ui.settings.recent_libraries = vec![gone.clone()];
+    let (checks, gate, calls) = slow_checks();
+    app.recent_checks = checks;
+    let mut h = Headless::new(app, [1300.0, 900.0], 1.0);
+    let enabled = |h: &Headless| match open_recent(&h.app).first() {
+        Some(MenuNode::Item { enabled, .. }) => *enabled,
+        other => panic!("{other:?}"),
+    };
+    for _ in 0..30 {
+        h.step();
+        assert!(enabled(&h), "not known yet: kept enabled (opening says why if it can't)");
+    }
+    release(&gate);
+    assert!(h.step_until(T, |h| !h.app.recent_checks.is_running() && h.app.recent_checks.is_known(&gone)), "answered");
+    for _ in 0..30 {
+        h.step();
+    }
+    assert!(!enabled(&h), "found missing: greyed out");
+    assert_eq!(calls.load(std::sync::atomic::Ordering::SeqCst), 1, "checked once, not every frame");
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
+/// The reasons are put into words on the UI thread, in the interface language (the worker that
+/// found them has none of its own).
+#[test]
+fn the_reasons_are_in_the_interface_language() {
+    let dir = temp("lang");
+    let gone = dir.join("Gone").to_string_lossy().into_owned();
+    let mut app = LightcraftApp::new(lightcraft_engine::Session::new().with_fs(), Services::default());
+    app.ui.language = crate::i18n::Locale::Fr;
+    app.ui.settings.recent_libraries = vec![gone.clone()];
+    app.library_problem = Some(LibraryProblem::choose_later(&app.ui.settings));
+    let mut h = Headless::new(app, [1300.0, 900.0], 1.0);
+    assert!(
+        h.step_until(T, |h| h.app.library_problem.as_ref().and_then(|p| p.chooser.as_ref()).is_some_and(|c| c.libraries.iter().all(|l| !l.checking)))
+    );
+    let c = h.app.library_problem.as_ref().and_then(|p| p.chooser.clone()).unwrap();
+    let why = c.libraries[0].unavailable.clone().unwrap_or_default();
+    let english = "Not found: moved, renamed or on a disconnected drive";
+    crate::i18n::set_language(crate::i18n::Locale::Fr);
+    assert_eq!(why, crate::i18n::tr(english), "translated on the UI thread");
+    assert_ne!(why, english, "the French catalog has this reason");
+    crate::i18n::set_language(crate::i18n::Locale::En);
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
+/// One drive that never answers doesn't hold the others: each path has its own worker, and the
+/// row that waits says the drive isn't answering once it takes long.
+#[test]
+fn one_hung_drive_doesnt_hold_the_other_libraries() {
+    let dir = temp("hung");
+    let a = make_library(&dir.join("A"));
+    let hung = dir.join("Hung").to_string_lossy().into_owned();
+    let gate = std::sync::Arc::new((std::sync::Mutex::new(false), std::sync::Condvar::new()));
+    let g = gate.clone();
+    let h_path = hung.clone();
+    let mut app = LightcraftApp::new(lightcraft_engine::Session::new().with_fs(), Services::default());
+    app.recent_checks = crate::panels::library_problem::RecentChecks::with_check(move |p| {
+        if p == h_path {
+            let (lock, cv) = &*g;
+            let open = lock.lock().unwrap();
+            let _unused = cv.wait_timeout_while(open, Duration::from_secs(10), |open| !*open).unwrap();
+        }
+        None
+    });
+    app.ui.settings.recent_libraries = vec![hung.clone(), a.clone()];
+    app.library_problem = Some(LibraryProblem::choose_later(&app.ui.settings));
+    let mut h = Headless::new(app, [1300.0, 900.0], 1.0);
+    let a_done = h.step_until(T, |h| h.app.library_problem.as_ref().and_then(|p| p.chooser.as_ref()).is_some_and(|c| !c.libraries[1].checking));
+    assert!(a_done, "the reachable library answered while the other one hangs");
+    let c = h.app.library_problem.as_ref().and_then(|p| p.chooser.clone()).unwrap();
+    assert!(c.libraries[0].checking, "the hung one is still checking");
+    release(&gate);
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
+/// A check whose worker dies (a panic) ends as "couldn't be checked" instead of leaving the row,
+/// and every later check, waiting forever.
+#[test]
+fn a_check_that_dies_doesnt_leave_the_list_waiting() {
+    let dir = temp("dies");
+    let a = make_library(&dir.join("A"));
+    let mut app = LightcraftApp::new(lightcraft_engine::Session::new().with_fs(), Services::default());
+    app.recent_checks = crate::panels::library_problem::RecentChecks::with_check(|_| panic!("a check that dies"));
+    app.ui.settings.recent_libraries = vec![a.clone()];
+    app.library_problem = Some(LibraryProblem::choose_later(&app.ui.settings));
+    let mut h = Headless::new(app, [1300.0, 900.0], 1.0);
+    let done = h.step_until(T, |h| h.app.library_problem.as_ref().and_then(|p| p.chooser.as_ref()).is_some_and(|c| !c.libraries[0].checking));
+    assert!(done, "the row stopped waiting");
+    assert!(!h.app.recent_checks.is_running(), "no check is left running");
+    let c = h.app.library_problem.as_ref().and_then(|p| p.chooser.clone()).unwrap();
+    assert!(c.libraries[0].unavailable.as_deref().is_some_and(|w| w.contains("couldn't be checked")), "{c:?}");
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
+/// A library found missing comes back once it is opened again (a drive plugged back in): the
+/// menu no longer greys it out.
+#[test]
+fn opening_a_library_clears_an_old_missing_answer() {
+    use crate::menubar::MenuNode;
+    let dir = temp("back");
+    let a = make_library(&dir.join("A"));
+    let mut app = LightcraftApp::new(lightcraft_engine::Session::new().with_fs(), Services::default());
+    app.ui.settings.recent_libraries = vec![a.clone()];
+    app.recent_checks.failed(&a, "the drive was disconnected");
+    let mut h = Headless::new(app, [1300.0, 900.0], 1.0);
+    h.app.run("app.openLibrary", json!({"path": a})).unwrap();
+    for _ in 0..4 {
+        h.step();
+    }
+    assert!(matches!(open_recent(&h.app).first(), Some(MenuNode::Item { enabled: true, .. })), "{:?}", open_recent(&h.app));
+    assert_eq!(h.app.recent_checks.reason(&a), None);
+    let _ = std::fs::remove_dir_all(&dir);
+}

crates/ui-egui/src/tests_switch_library.rs

@@ -163,6 +163,10 @@ fn missing_recent_libraries_are_greyed_out_and_pruned_and_the_list_is_capped() {
     let mut app = app_in(&dir);
     let gone = dir.join("gone").to_string_lossy().into_owned();
     app.ui.settings.recent_libraries = vec![gone.clone()];
+    // the menu takes what a background check found (issue #1290 review): let it answer first
+    let ctx = egui::Context::default();
+    crate::panels::library_problem::logic(&mut app, &ctx);
+    assert!(crate::tasks::wait(&mut app, &ctx, std::time::Duration::from_secs(20)), "the check answered");
     let disabled = recent_submenu(&app)
         .iter()
         .any(|n| matches!(n, crate::menubar::MenuNode::Item { params, enabled: false, .. } if params["path"] == gone.as_str()));

docs/control-protocol.md

@@ -173,17 +173,24 @@ With Settings ▸ General ▸ At launch set to Ask me (`ui.set {"settings": {"at
 default, opens the last library), or Alt/Option held during the first 2 s after the window appears, the desktop app
 shows a window listing the recent libraries (issue #1290). `--library` and `LIGHTCRAFT_LIBRARY` skip it, and so does
 `--memory`; `lightcraft-cli` never shows it. `ui.inspect` → `libraryProblem.chooser` is `{libraries: [{path,
-lastOpened, available, reason}], selected, libraryOpen}` (`available` is checked read-only: the folder and its catalog
-file are looked at, never locked or written, so a library another program has open shows as available and Open
-reports who has it in `libraryProblem.error`) (else `null`; `libraryOpen: true` when it was opened over
-the last library with Alt). Each row is the widget `libraryRecent:<index>`; the buttons are `button:libraryOpen`,
+lastOpened, available, checking, reason}], selected, libraryOpen}` (else `null`; `libraryOpen: true` when it was opened
+over the last library with Alt). Each library is checked read-only on a worker thread of its own, never on the UI
+thread (the folder and its catalog file are looked at, never locked or written, so a library another program has
+open shows as available and Open reports who has it in `libraryProblem.error`). `checking: true` until its check
+answers (the row says *Checking…*, then that the drive isn't answering after 5 s); `available` is `false` only
+once a check found a `reason`, so a row still checking can be opened. The selection starts on the last library and
+moves to the first one that can be opened once every check has answered, unless it was moved meanwhile. Each row is the widget `libraryRecent:<index>`; the buttons are `button:libraryOpen`,
 `button:libraryOpenAnother` (the folder dialog of `app.openLibrary`), `button:libraryNew` (`app.newLibrary`),
 `button:libraryForget` (Remove from List) and `button:libraryQuit`; `ui.key` ArrowUp / ArrowDown move the selection,
 Enter opens it and Escape quits (over an open library: closes the window and keeps it). `app.openLibrary {path}`
 opening a library ends it. While it shows, the settings are saved as usual (a removal is kept).
 
-`app.recentLibraries` returns `[{path, name, exists, open, lastOpened}]` (`lastOpened`: ISO 8601, `null` when not
-known; kept in `ui.json` → `settings.recentLibrariesOpened`). `{remove: path}` forgets one entry (an error if the list
+`app.recentLibraries` returns `[{path, name, exists, checking, open, lastOpened}]`. `exists` is what the last
+background check found (`false`: moved, on a disconnected drive or no longer a library; `null`: not checked yet) and
+`checking` whether one is running: the command never looks at a folder itself. Recent libraries are checked in the
+background when the app starts, again after 30 s, and when the launch window opens; File ▸ Open Recent Library
+greys out only an entry a check found missing. `lastOpened`: ISO 8601, `null` when not known; kept in `ui.json` →
+`settings.recentLibrariesOpened`. `{remove: path}` forgets one entry (an error if the list
 doesn't hold it) and `{clear: true}` (or `app.clearRecentLibraries`) all of them; nothing on disk is touched.
 
 ### Open Library on a Lightroom catalog

docs/whats-new.md

@@ -6,7 +6,9 @@
 - Settings ▸ General ▸ **At launch**: *Open the last library* (as before) or *Ask me*. With *Ask me*, LightCraft
   starts with a window listing your recent libraries: each one's name, the folder it is in and when you last opened
   it. One that is gone (moved, renamed, on a disconnected drive), can't be read or no longer holds a library is
-  greyed out with the reason. Making the list only looks: it never locks or writes anything in your libraries, so one
+  greyed out with the reason. Each one is checked in the background, showing *Checking…* until it answers (and that
+  the drive isn't answering after 5 s), so the list never waits on a network share that is offline or a disk that
+  has to wake up; File ▸ Open Recent Library uses the same answers. Making the list only looks: it never locks or writes anything in your libraries, so one
   that another program has open is reported, with who has it, when you press **Open**, and the window stays. The
   last library is selected; **Open** (or Return, or a double-click) opens it,
   **Open Another…** asks for a folder, **New Library…** makes one, **Quit** (or Esc) quits. ↑ and ↓ move through