refactor: fit view — pure CSS layout, no DOM measurement in applyFitLayout

Root cause of repeated fit view failures: applyFitLayout() measured
clientHeight/clientWidth/getComputedStyle and set inline tile heights. This
failed in multiple ways:
  - clientHeight = 0 when container is display:none (returning from session)
  - inline tile.style.height destroyed every 2s by innerHTML rebuild in pollSessions
  - getComputedStyle forces style recalc with potentially stale values
  - rAF wrappers couldn't reliably fix timing issues

Fix: pure arithmetic applyFitLayout — no DOM measurement, no inline heights.
The grid already has a definite height from CSS (flex:1 inside height:100dvh).
grid-template-rows: repeat(rows, 1fr) divides that space without JS measurement.

CSS changes:
  - .session-grid--fit { align-content: stretch }
  - .session-grid--fit .session-tile { height: auto } — CSS grid cell controls sizing

JS changes:
  - applyFitLayout() removes all clientHeight/clientWidth/getComputedStyle/style.height
  - applyFitLayout() is now pure arithmetic: count tiles, compute cols×rows, set 1fr templates
  - Removed rAF wrappers from all call sites (renderGrid, closeSession, resize handler,
    applyDisplaySettings) — safe to call synchronously since no layout measurement needed
  - applyDisplaySettings() no longer clears tile heights (never set them anymore)

Tests updated:
  - Added test_fit_view_session_tile_has_height_auto (CSS)
  - Added 'applyFitLayout does NOT measure DOM dimensions' (JS)
  - Updated 'applyFitLayout clears stale...' → 'sets gridTemplateColumns/Rows via arithmetic'
  - Updated rAF-wrapper test to match new direct-call behavior

muxplex/frontend/app.js

@@ -654,10 +654,7 @@ function renderGrid(sessions) {
   var currentMode = currentDs.viewMode || 'auto';
   if (currentMode === 'fit' && grid) {
     grid.classList.add('session-grid--fit');
-    requestAnimationFrame(function() {
     applyFitLayout(grid);
-      // No scrollTop hack needed — CSS flex + justify-content:flex-end anchors content to bottom
-    });
   }
 
 }
@@ -1014,7 +1011,7 @@ function closeSession() {
     var _closGrid = document.getElementById('session-grid');
     if (_closGrid) {
       _closGrid.classList.add('session-grid--fit');
-      requestAnimationFrame(function() { applyFitLayout(_closGrid); });
+      applyFitLayout(_closGrid);
     }
   }
 
@@ -1098,36 +1095,24 @@ function saveDisplaySettings(settings) {
 }
 
 /**
- * Calculate and apply grid layout to fill the viewport exactly (Fit mode).
+ * Set grid template for fit mode based on tile count.
- * Determines optimal cols × rows based on tile count and available space.
+ * Pure arithmetic — no DOM measurement, no getComputedStyle, no clientHeight.
+ * Safe to call at any time regardless of display state or layout phase.
+ *
+ * The grid already has a definite height from CSS (flex: 1 inside height: 100dvh).
+ * Setting grid-template-rows: repeat(rows, 1fr) lets the browser divide that height
+ * equally without JS needing to know the pixel dimensions.  Tiles use height: auto
+ * (set in CSS) so they fill their grid cells without inline style overrides.
+ *
  * @param {Element} grid - The session grid element
  */
 function applyFitLayout(grid) {
-  // Clear stale layout from previous calls (prevents interference on page reload and
-  // when returning from a session where the grid was display:none during measurement)
-  grid.style.removeProperty('grid-template-rows');
-  grid.querySelectorAll('.session-tile').forEach(function(t) {
-    t.style.removeProperty('height');
-  });
-
   var count = grid.querySelectorAll('.session-tile').length;
-  if (count === 0) return;
+  if (count === 0) {
-
+    grid.style.removeProperty('grid-template-columns');
-  // Available space — use grid's parent container
+    grid.style.removeProperty('grid-template-rows');
-  var parent = grid.parentElement;
+    return;
-  var availH = parent ? parent.clientHeight : window.innerHeight;
+  }
-  var availW = grid.clientWidth;
-
-  // Subtract padding and gap
-  var style = getComputedStyle(grid);
-  var padT = parseFloat(style.paddingTop) || 0;
-  var padB = parseFloat(style.paddingBottom) || 0;
-  var padL = parseFloat(style.paddingLeft) || 0;
-  var padR = parseFloat(style.paddingRight) || 0;
-  var gap = parseFloat(style.gap) || 8;
-
-  var innerW = availW - padL - padR;
-  var innerH = availH - padT - padB;
 
   // Calculate optimal cols/rows — start with square root
   var cols = Math.ceil(Math.sqrt(count));
@@ -1143,16 +1128,8 @@ function applyFitLayout(grid) {
     }
   }
 
-  // Tile height from available space
-  var tileH = (innerH - gap * (rows - 1)) / rows;
-
   grid.style.gridTemplateColumns = 'repeat(' + cols + ', 1fr)';
   grid.style.gridTemplateRows = 'repeat(' + rows + ', 1fr)';
-
-  // Override tile height so tiles fill the grid rows
-  grid.querySelectorAll('.session-tile').forEach(function(t) {
-    t.style.height = tileH + 'px';
-  });
 }
 
 /**
@@ -1199,9 +1176,7 @@ function applyDisplaySettings(ds) {
 
   // Reset any inline styles from previous fit calculation
   grid.style.removeProperty('grid-template-rows');
-  grid.querySelectorAll('.session-tile').forEach(function(t) {
+  grid.style.removeProperty('grid-template-columns');
-    t.style.removeProperty('height');
-  });
 
   if (mode === 'auto') {
     // Restore grid columns setting
@@ -1213,7 +1188,7 @@ function applyDisplaySettings(ds) {
 
   } else if (mode === 'fit') {
     grid.classList.add('session-grid--fit');
-    requestAnimationFrame(function() { applyFitLayout(grid); });
+    applyFitLayout(grid);
   }
 }
 
@@ -1880,7 +1855,7 @@ window.addEventListener('resize', function() {
   var ds = loadDisplaySettings();
   if ((ds.viewMode || 'auto') === 'fit') {
     var grid = document.getElementById('session-grid');
-    if (grid) requestAnimationFrame(function() { applyFitLayout(grid); });
+    if (grid) applyFitLayout(grid);
   }
 });
 

muxplex/frontend/style.css

@@ -1380,6 +1380,15 @@ body {
 /* Fit view — tiles fill viewport, no scroll */
 .session-grid--fit {
   overflow: hidden;
+  align-content: stretch;   /* let 1fr rows fill container height */
+}
+
+/* In fit mode, tiles must use height:auto so the CSS grid 1fr rows control sizing.
+   The base .session-tile uses var(--tile-height) which is a fixed pixel value —
+   overriding with height:auto lets the grid cell (from grid-template-rows: repeat(n,1fr))
+   determine the tile height instead. JS sets only grid-template-columns/rows, not inline heights. */
+.session-grid--fit .session-tile {
+  height: auto;
 }
 
 /* In fit mode, tile body and pre use the base CSS:

muxplex/frontend/tests/test_app.mjs

@@ -2205,11 +2205,22 @@ test('bindStaticEventListeners wires view-mode-btn click to cycleViewMode', () =
   );
 });
 
-test('applyFitLayout is called via requestAnimationFrame for correct timing', () => {
+test('applyFitLayout is called directly (no requestAnimationFrame needed — pure arithmetic)', () => {
+  // Pure arithmetic applyFitLayout is safe to call synchronously at any time —
+  // it does not measure DOM dimensions so there is no layout-timing dependency.
+  // Call sites (renderGrid, closeSession, applyDisplaySettings, resize handler)
+  // should call it directly, not via requestAnimationFrame.
   const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  // Verify applyFitLayout exists as a direct call (not only inside rAF callbacks)
   assert.ok(
-    source.includes('requestAnimationFrame') && source.includes('applyFitLayout'),
+    source.includes('applyFitLayout'),
-    'applyFitLayout must be deferred via requestAnimationFrame'
+    'app.js must define and call applyFitLayout'
+  );
+  // The function body itself must not measure DOM — verified by the separate
+  // "does NOT measure DOM dimensions" test. Here just confirm the function exists.
+  assert.ok(
+    source.includes('function applyFitLayout('),
+    'applyFitLayout function must be declared'
   );
 });
 
@@ -2256,48 +2267,62 @@ test('CSS style.css uses base position:absolute bottom:0 for fit mode content an
   );
 });
 
-// --- Fit view Bug 1: applyFitLayout clears stale layout before recalculating ---
+// --- Fit view layout: applyFitLayout sets grid template via pure arithmetic ---
 
-test('applyFitLayout clears stale grid-template-rows and tile heights before recalculating', () => {
+test('applyFitLayout sets gridTemplateColumns and gridTemplateRows via pure arithmetic', () => {
-  const removedGridProps = [];
+  const assignedProps = {};
-  const removedTileProps = [];
 
-  const mockTile = {
+  const mockTile = { style: { removeProperty: () => {} } };
-    style: { removeProperty: (prop) => removedTileProps.push(prop) },
-  };
 
   const mockGrid = {
-    style: {
+    style: new Proxy(assignedProps, {
-      removeProperty: (prop) => removedGridProps.push(prop),
+      set(target, prop, value) { target[prop] = value; return true; },
-      gridTemplateColumns: '',
+      get(target, prop) { return target[prop]; },
-      gridTemplateRows: 'repeat(2, 1fr)',  // stale value from previous call
+    }),
-    },
     querySelectorAll: (sel) => {
-      if (sel === '.session-tile') return [mockTile];
+      if (sel === '.session-tile') return [mockTile, mockTile, mockTile, mockTile];
       return [];
     },
-    clientWidth: 800,
-    parentElement: { clientHeight: 600 },
   };
 
-  const origGetComputedStyle = globalThis.getComputedStyle;
-  globalThis.getComputedStyle = () => ({
-    paddingTop: '16px', paddingBottom: '16px',
-    paddingLeft: '16px', paddingRight: '16px',
-    gap: '8px',
-  });
-
   app.applyFitLayout(mockGrid);
 
   assert.ok(
-    removedGridProps.includes('grid-template-rows'),
+    typeof assignedProps.gridTemplateColumns === 'string' && assignedProps.gridTemplateColumns.includes('1fr'),
-    'applyFitLayout must removeProperty("grid-template-rows") before recalculating to prevent stale layout interference'
+    'applyFitLayout must set gridTemplateColumns with 1fr tracks'
   );
   assert.ok(
-    removedTileProps.includes('height'),
+    typeof assignedProps.gridTemplateRows === 'string' && assignedProps.gridTemplateRows.includes('1fr'),
-    'applyFitLayout must removeProperty("height") from each tile before recalculating'
+    'applyFitLayout must set gridTemplateRows with 1fr tracks'
   );
-
+});
-  if (origGetComputedStyle) globalThis.getComputedStyle = origGetComputedStyle;
+
-  else delete globalThis.getComputedStyle;
+// --- Pure CSS fit layout: applyFitLayout must not measure DOM ---
+
+test('applyFitLayout does NOT measure DOM dimensions (pure arithmetic)', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  const fnStart = source.indexOf('function applyFitLayout(');
+  assert.ok(fnStart !== -1, 'applyFitLayout function must exist');
+  // Find end of function: next \n} at the same nesting level
+  let depth = 0;
+  let pos = source.indexOf('{', fnStart);
+  const fnBodyStart = pos;
+  while (pos < source.length) {
+    if (source[pos] === '{') depth++;
+    else if (source[pos] === '}') {
+      depth--;
+      if (depth === 0) break;
+    }
+    pos++;
+  }
+  const fnBody = source.substring(fnBodyStart, pos + 1);
+
+  assert.ok(!fnBody.includes('clientHeight'),
+    'applyFitLayout must NOT read clientHeight — causes wrong values when container is display:none');
+  assert.ok(!fnBody.includes('clientWidth'),
+    'applyFitLayout must NOT read clientWidth — pure 1fr CSS handles width');
+  assert.ok(!fnBody.includes('getComputedStyle'),
+    'applyFitLayout must NOT call getComputedStyle — pure 1fr CSS handles gap/padding');
+  assert.ok(!fnBody.includes('.style.height'),
+    'applyFitLayout must NOT set inline tile heights — CSS grid 1fr rows handle sizing');
 });

muxplex/tests/test_frontend_css.py

@@ -1762,6 +1762,28 @@ def test_fit_view_no_tile_body_flex_override() -> None:
     )
 
 
+def test_fit_view_session_tile_has_height_auto() -> None:
+    """Pure CSS fit layout: .session-grid--fit .session-tile must use height: auto.
+
+    JS was measuring clientHeight and setting tile.style.height = tileH + 'px'.
+    This failed when the grid was display:none (clientHeight = 0) and after
+    innerHTML rebuilds destroyed inline styles every 2s.
+
+    Pure CSS fix: the grid has a definite height (flex:1 inside 100dvh).
+    grid-template-rows: repeat(rows, 1fr) divides that height equally.
+    height: auto on tiles lets them fill their grid cells without JS measurement.
+    """
+    css = read_css()
+    assert ".session-grid--fit .session-tile {" in css, (
+        ".session-grid--fit .session-tile rule must exist for pure CSS fit layout"
+    )
+    block = _extract_rule_block(css, ".session-grid--fit .session-tile {")
+    assert "height: auto" in block or "height:auto" in block, (
+        ".session-grid--fit .session-tile must use height: auto — "
+        "JS inline height setting was unreliable (lost on innerHTML rebuild every 2s)"
+    )
+
+
 def test_fit_view_no_pre_static_override() -> None:
     """Bug fix: .session-grid--fit .tile-body pre must NOT override position to static.