fix: fit view — defer layout measurement to rAF, stretch pre to fill tile

Bug 1: applyFitLayout measured parentElement.clientHeight before the
browser reflowed the DOM — returned 0 on first render. Fix: wrap in
requestAnimationFrame so measurement happens after layout pass.
Applies to all three call sites: renderGrid(), applyDisplaySettings(),
and the window resize handler.

Bug 2: .tile-body pre had position:absolute bottom:0 but no top:0.
In fit mode (taller tiles), content sat at the bottom with a gap above.
Fix: .session-grid--fit .tile-body pre { top: 0; overflow-y: auto }

muxplex/frontend/app.js

@@ -652,7 +652,7 @@ function renderGrid(sessions) {
   var currentMode = currentDs.viewMode || 'auto';
   if (currentMode === 'fit' && grid) {
     grid.classList.add('session-grid--fit');
-    applyFitLayout(grid);
+    requestAnimationFrame(function() { applyFitLayout(grid); });
   }
 
 }
@@ -1191,7 +1191,7 @@ function applyDisplaySettings(ds) {
 
   } else if (mode === 'fit') {
     grid.classList.add('session-grid--fit');
-    applyFitLayout(grid);
+    requestAnimationFrame(function() { applyFitLayout(grid); });
   }
 }
 
@@ -1858,7 +1858,7 @@ window.addEventListener('resize', function() {
   var ds = loadDisplaySettings();
   if ((ds.viewMode || 'auto') === 'fit') {
     var grid = document.getElementById('session-grid');
-    if (grid) applyFitLayout(grid);
+    if (grid) requestAnimationFrame(function() { applyFitLayout(grid); });
   }
 });
 

muxplex/frontend/style.css

@@ -1380,6 +1380,12 @@ body {
   overflow: hidden;
 }
 
+/* In fit mode, stretch pre to fill the full tile body height (not just bottom-anchor) */
+.session-grid--fit .tile-body pre {
+  top: 0;           /* stretch to fill full tile body height in fit mode */
+  overflow-y: auto; /* scroll if content exceeds the tile */
+}
+
 /* ============================================================
    Responsive overlay sidebar at <960px
    ============================================================ */

muxplex/frontend/tests/test_app.mjs

@@ -2204,3 +2204,11 @@ test('bindStaticEventListeners wires view-mode-btn click to cycleViewMode', () =
     'bindStaticEventListeners must wire #view-mode-btn click handler'
   );
 });
+
+test('applyFitLayout is called via requestAnimationFrame for correct timing', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  assert.ok(
+    source.includes('requestAnimationFrame') && source.includes('applyFitLayout'),
+    'applyFitLayout must be deferred via requestAnimationFrame'
+  );
+});