feat: hover preview popover on dashboard tiles

Desktop only (guarded by ontouchstart check). On mouseenter of a
session tile, shows a floating popover with the full tmux snapshot
(all lines, not the cropped 20-line preview). Positioned right of
tile if space, else left. 100ms delay prevents flicker on brush-over.
Data from _currentSessions (already in memory, zero API calls).

Also calls hidePreview() in openSession() so the popover is cleaned
up when switching to fullscreen view.

muxplex/frontend/app.js

@@ -125,6 +125,8 @@ let _pollingTimer;
 let _heartbeatTimer;
 let _notificationPermission = 'default';
 let _pollFailCount = 0;
+let _previewPopover = null;
+let _previewTimer = null;
 
 // ─── DOM helpers ──────────────────────────────────────────────────────────────
 function $(id) {
@@ -509,6 +511,62 @@ function renderGrid(sessions) {
   }
 }
 
+// ---------------------------------------------------------------------------
+// Hover preview popover (desktop only — no hover on touch devices)
+// ---------------------------------------------------------------------------
+
+function showPreview(tile) {
+  var name = tile.dataset.session;
+  if (!name || !_currentSessions) return;
+
+  var session = _currentSessions.find(function (s) { return s.name === name; });
+  if (!session || !session.snapshot) return;
+
+  hidePreview();
+
+  var popover = document.createElement('div');
+  popover.className = 'preview-popover';
+  var pre = document.createElement('pre');
+  pre.textContent = session.snapshot;
+  popover.appendChild(pre);
+  document.body.appendChild(popover);
+
+  // Position: right of tile if room, else left
+  var rect = tile.getBoundingClientRect();
+  var popW = popover.offsetWidth;
+  var popH = popover.offsetHeight;
+  var left, top;
+
+  if (rect.right + popW + 12 < window.innerWidth) {
+    left = rect.right + 8;
+  } else {
+    left = rect.left - popW - 8;
+  }
+
+  // Vertically: align top with tile, clamp to viewport
+  top = rect.top;
+  if (top + popH > window.innerHeight - 16) {
+    top = window.innerHeight - popH - 16;
+  }
+  if (top < 8) top = 8;
+
+  popover.style.left = left + 'px';
+  popover.style.top = top + 'px';
+
+  _previewPopover = popover;
+}
+
+function hidePreview() {
+  if (_previewTimer) {
+    clearTimeout(_previewTimer);
+    _previewTimer = null;
+  }
+  if (_previewPopover) {
+    _previewPopover.remove();
+    _previewPopover = null;
+  }
+}
+
 // ─── Notification permission ────────────────────────────────────────────────
 
 /**
@@ -627,6 +685,7 @@ function updatePillBell() {
  * @returns {Promise<void>}
  */
 async function openSession(name, opts = {}) {
+  hidePreview();
   _viewingSession = name;
   _viewMode = 'fullscreen';
 
@@ -1010,6 +1069,22 @@ function bindStaticEventListeners() {
   document.addEventListener('keydown', handleGlobalKeydown);
   on($('session-pill'), 'click', openBottomSheet);
   on($('sheet-backdrop'), 'click', closeBottomSheet);
+
+  // Hover preview — delegated on grid container (tiles are re-rendered each poll)
+  var gridEl = $('session-grid');
+  if (gridEl && !('ontouchstart' in window)) {  // desktop only
+    gridEl.addEventListener('mouseenter', function (e) {
+      var tile = e.target.closest('.session-tile');
+      if (!tile) return;
+      _previewTimer = setTimeout(function () { showPreview(tile); }, 100);
+    }, true);  // useCapture: true for delegation with mouseenter
+
+    gridEl.addEventListener('mouseleave', function (e) {
+      var tile = e.target.closest('.session-tile');
+      if (!tile) return;
+      hidePreview();
+    }, true);
+  }
 }
 
 // ─── Test-only helpers ────────────────────────────────────────────────────────
@@ -1117,6 +1192,9 @@ if (typeof module !== 'undefined' && module.exports) {
     closeBottomSheet,
     renderSheetList,
     updateSessionPill,
+    // Hover preview popover
+    showPreview,
+    hidePreview,
     // Test-only helpers
     _setCurrentSessions,
     _setPaletteFilteredSessions,

muxplex/frontend/style.css

@@ -959,6 +959,35 @@ body {
   display: block;
 }
 
+/* ============================================================
+   Hover preview popover (desktop dashboard)
+   ============================================================ */
+
+.preview-popover {
+  position: fixed;
+  z-index: 500;
+  background: var(--bg-secondary);
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  padding: 10px 12px;
+  min-width: 480px;
+  max-width: 640px;
+  max-height: 80vh;
+  overflow-y: auto;
+  box-shadow: 0 8px 32px rgba(0, 0, 0, 0.5);
+  pointer-events: none;   /* don't interfere with mouse leaving tile */
+}
+
+.preview-popover pre {
+  margin: 0;
+  font-family: var(--font-mono);
+  font-size: 11px;
+  line-height: 1.4;
+  color: var(--text-muted);
+  white-space: pre;
+  overflow-x: hidden;
+}
+
 /* ============================================================
    Responsive overlay sidebar at <960px
    ============================================================ */

muxplex/frontend/tests/test_app.mjs

@@ -2050,4 +2050,16 @@ test('openSession mounts terminal AFTER connect POST, not inside animation timer
     '_openTerminal must appear AFTER the /connect POST in the source');
 });
 
+// --- Hover preview popover ---
+
+test('app.js has hover preview popover with desktop-only guard', () => {
+  const source = fs.readFileSync(
+    new URL('../app.js', import.meta.url), 'utf8'
+  );
+  assert.ok(source.includes('preview-popover'), 'must create popover with preview-popover class');
+  assert.ok(source.includes('ontouchstart'), 'must guard against touch devices (desktop only)');
+  assert.ok(source.includes('session.snapshot'), 'must use full snapshot text (not lastLines)');
+  assert.ok(source.includes('hidePreview'), 'must have cleanup on mouseleave');
+});
+
 

muxplex/tests/test_frontend_css.py

@@ -613,3 +613,21 @@ def test_sidebar_item_has_flex_shrink_zero():
     assert "flex-shrink: 0" in block, (
         ".sidebar-item must have flex-shrink:0 to prevent compression"
     )
+
+
+# ============================================================
+# Hover preview popover (desktop dashboard)
+# ============================================================
+
+
+def test_preview_popover_css_exists():
+    """Preview popover must have CSS rules with position: fixed."""
+    css = read_css()
+    assert ".preview-popover" in css, "must have .preview-popover CSS class"
+    popover_idx = css.index(".preview-popover")
+    block_start = css.index("{", popover_idx)
+    block_end = css.index("}", block_start)
+    block = css[block_start:block_end]
+    assert "position: fixed" in block or "position:fixed" in block, (
+        ".preview-popover must use position: fixed"
+    )