connect: leaving fullscreen keeps the session, with a button back

The launcher tore the session down whenever the tab left fullscreen, so
holding Escape, F11 or a gesture from the OS ended it. Leaving fullscreen
now keeps the session at the tab's size and shows a Fullscreen button
that goes back; only Exit, or the session ending, closes it. Tested with
the browser's fullscreen events faked, since headless Chromium refuses
the real request.

static/rustguac.css

@@ -1141,12 +1141,15 @@ html:not([data-role-level="4"]) .admin-only { display: none !important; }
     border: 3px solid rgba(255,255,255,0.25); border-top-color: var(--accent, #5bc0be);
     border-radius: 50%; animation: rg-btn-spin 0.8s linear infinite;
 }
-#rg-session-exit {
-    position: absolute; top: 10px; right: 14px; z-index: 5001;
+#rg-session-exit { position: absolute; top: 10px; right: 14px; }
+#rg-session-fs { position: absolute; top: 10px; right: 104px; }
+#rg-session-fs[hidden] { display: none; }
+.rg-session-btn {
+    z-index: 5001;
     height: 32px; padding: 0 var(--s-3, 14px);
     background: rgba(20,24,38,0.72); color: var(--text, #e0e0e0);
     border: 1px solid var(--border, #333); border-radius: var(--radius, 2px);
     font-family: monospace; font-size: var(--fz-sm, 16px); cursor: pointer;
     opacity: 0.35; transition: opacity 0.15s;
 }
-#rg-session-exit:hover { opacity: 1; background: var(--surface, #16213e); border-color: var(--primary, #e94560); color: var(--text, #e0e0e0); }
+.rg-session-btn:hover { opacity: 1; background: var(--surface, #16213e); border-color: var(--primary, #e94560); color: var(--text, #e0e0e0); }

static/search.js

@@ -422,7 +422,8 @@
         } else fsRefused();
         var ov = document.createElement('div'); ov.id = 'rg-session-overlay';
         ov.innerHTML = '<div id="rg-session-spinner"></div>' +
-            '<button id="rg-session-exit" type="button" title="Exit">× Exit</button>' +
+            '<button id="rg-session-fs" class="rg-session-btn" type="button" title="Back to fullscreen" hidden>⛶ Fullscreen</button>' +
+            '<button id="rg-session-exit" class="rg-session-btn" type="button" title="Leave the session">× Exit</button>' +
             '<iframe id="rg-session-frame" allow="fullscreen; clipboard-read; clipboard-write" title="Session"></iframe>';
         document.body.appendChild(ov);
         if (btn) { btn.classList.add('loading'); btn.disabled = true; }
@@ -450,11 +451,23 @@
                 // the settle check covers both.
                 if (!started) armSettle();
             }
-            else if (enteredFs) { teardown(); }
+            // Leaving fullscreen (Escape held, F11, the OS) keeps the session:
+            // it carries on at the tab's size and the button takes it back
+            // to fullscreen. Only Exit, or the session ending, closes it.
+            else if (enteredFs) { showFsButton(true); }
+            if (document.fullscreenElement) showFsButton(false);
+        }
+        function showFsButton(on) {
+            var b = document.getElementById('rg-session-fs'); if (b) b.hidden = !on;
+            if (!on) { var f = document.getElementById('rg-session-frame'); try { if (f) f.focus(); } catch (e) {} }
         }
         window.addEventListener('message', onMsg);
         document.addEventListener('fullscreenchange', onFsChange);
         document.getElementById('rg-session-exit').addEventListener('click', teardown);
+        document.getElementById('rg-session-fs').addEventListener('click', function () {
+            // A click is a fresh gesture, so the browser allows fullscreen again.
+            try { var p = req && req.call(el); if (p && p.catch) p.catch(function () {}); } catch (e) {}
+        });
         // Create the session at the measured viewport size — the size the
         // frame paints into — so there is nothing to resize afterwards.
         function startConnect() {

tests/e2e/specs/zero-click-fullscreen.spec.mjs

@@ -55,3 +55,50 @@ test('a non-fullscreen entry connects the old way, with no frame', async ({ page
     expect(await page.locator('#rg-session-overlay').count()).toBe(0);
     if (popup) await popup.close();
 });
+
+// Leaving fullscreen (Escape held, F11, the OS) must not end the session: the
+// frame stays at the tab's size and a button takes it back to fullscreen.
+// Headless Chromium refuses real fullscreen, so the browser's side is faked:
+// fullscreenElement is set and cleared and fullscreenchange fired, exactly
+// what the launcher listens to.
+test('leaving fullscreen keeps the session and offers to go back', async ({ page }) => {
+    await page.addInitScript(() => {
+        let fs = null;
+        Object.defineProperty(Document.prototype, 'fullscreenElement', { configurable: true, get: () => fs });
+        // Like a real browser, the change event fires after the call returns.
+        const change = (el) => new Promise((resolve) => setTimeout(() => {
+            fs = el;
+            document.dispatchEvent(new Event('fullscreenchange'));
+            resolve();
+        }, 50));
+        Element.prototype.requestFullscreen = function () { return change(document.documentElement); };
+        document.exitFullscreen = function () { return change(null); };
+        window.__leaveFullscreen = () => document.exitFullscreen();
+    });
+    await installApi(page);
+    await page.goto('/connections.html');
+    await settle(page);
+    await page.locator('.folder-row').filter({ hasText: /acme/ }).first().click();
+    const kiosk = page.locator('.entries-table tbody tr').filter({ hasText: 'kiosk' });
+    await kiosk.locator('button', { hasText: /^Connect/ }).first().click();
+
+    const overlay = page.locator('#rg-session-overlay');
+    const back = page.locator('#rg-session-fs');
+    await expect(overlay).toBeVisible();
+    await expect(back).toBeHidden();
+
+    // The user leaves fullscreen: the session is still there.
+    await page.evaluate(() => window.__leaveFullscreen());
+    await expect(overlay).toBeVisible();
+    await expect(page.locator('#rg-session-frame')).toHaveCount(1);
+    await expect(back).toBeVisible();
+
+    // The button goes back to fullscreen and hides itself.
+    await back.click();
+    await expect(back).toBeHidden();
+    await expect(overlay).toBeVisible();
+
+    // Exit still ends it.
+    await page.locator('#rg-session-exit').click();
+    await expect(overlay).toHaveCount(0);
+});