fix: hover preview flashing — skip grid/sidebar re-render while preview active

Poll cycle rebuilds innerHTML every ~2s, destroying the hovered tile's
DOM element and tile--previewing class. Fix: renderGrid() and
renderSidebar() early-return when _previewPopover is non-null. On
hidePreview(), a catch-up render fires so no data is stale.

muxplex/frontend/app.js

@@ -353,6 +353,7 @@ function buildSidebarHTML(session, currentSession) {
  */
 function renderSidebar(sessions, currentSession) {
   if (_viewMode !== 'fullscreen') return;
+  if (_previewPopover) return;  // skip re-render while hover preview is active
 
   const list = $('sidebar-list');
   if (!list) return;
@@ -482,6 +483,8 @@ function bindSidebarClickAway() {
  * @param {object[]} sessions
  */
 function renderGrid(sessions) {
+  if (_previewPopover) return;  // skip re-render while hover preview is active
+
   const grid = $('session-grid');
   const emptyState = $('empty-state');
 
@@ -612,6 +615,11 @@ function hidePreview() {
     _previewEl.classList.remove('tile--previewing', 'item--previewing');
     _previewEl = null;
   }
+  // Catch-up render after preview is dismissed — apply any data missed while paused
+  if (_currentSessions) {
+    renderGrid(_currentSessions);
+    renderSidebar(_currentSessions, _viewingSession);
+  }
 }
 
 // ─── Notification permission ────────────────────────────────────────────────

muxplex/frontend/tests/test_app.mjs

@@ -2091,4 +2091,22 @@ test('hover preview delay is 1500ms (not 350ms)', () => {
   assert.ok(!source.includes(', 350)'), 'old 350ms delay must be removed');
 });
 
+test('renderGrid and renderSidebar skip re-render while preview is active', () => {
+  const source = fs.readFileSync(
+    new URL('../app.js', import.meta.url), 'utf8'
+  );
+
+  // Find renderGrid function body
+  const gridFnStart = source.indexOf('function renderGrid');
+  const gridFnBody = source.substring(gridFnStart, gridFnStart + 500);
+  assert.ok(gridFnBody.includes('_previewPopover'),
+    'renderGrid must check _previewPopover and skip while active');
+
+  // Find renderSidebar function body
+  const sidebarFnStart = source.indexOf('function renderSidebar');
+  const sidebarFnBody = source.substring(sidebarFnStart, sidebarFnStart + 500);
+  assert.ok(sidebarFnBody.includes('_previewPopover'),
+    'renderSidebar must check _previewPopover and skip while active');
+});
+