refactor(ui): share a short right rail strip in CSS alone

The fitTabs pass and its lane observer go: the column stands the lane's full height, which makes the strip's size definite, and the tabs take flex 1 1 0% with max-height max-content, so flexbox hands the strip's height out in equal parts, a tab's own length caps its part, and what a short label leaves goes to the longer ones. The basis is 0%, not 0, so the strip's content size, which sizes the column, still comes from the labels. Only the strip and the tool slot take the pointer; the measuring pass sets the column back to height auto so the rail's climb sees the natural column. Same heights as the JS pass at every viewport checked.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/ui/rightRailTabs.js

@@ -296,69 +296,12 @@ export function createRightRailTabs({
     });
   };
 
-  /** A length from a computed style, in pixels. */
+  /** A join token from the stylesheet (--right-rail-*), in pixels. */
   const token = (computed, name, fallback) => {
     const value = parseFloat(computed?.getPropertyValue?.(name));
     return Number.isFinite(value) ? value : fallback;
   };
 
-  /**
-   * Share a strip shorter than its tabs. The lane (--right-stack-max-height)
-   * less the tool slot and the strip's gaps is divided equally; a tab shorter
-   * than its share keeps its height and the rest is shared again, so the long
-   * labels end up the same height and clip alike (styles/right-rail-tabs.css).
-   * Tabs that fit keep their natural height.
-   */
-  const heights = new WeakMap();
-  const setHeight = (tab, value) => {
-    if (heights.get(tab) === value) return;
-    heights.set(tab, value);
-    if (value) tab.style.setProperty('height', value);
-    else tab.style.removeProperty('height');
-  };
-  function fitTabs(tabs) {
-    const styleOf = (el) => documentRef.defaultView?.getComputedStyle?.(el);
-    const stripStyle = styleOf(strip);
-    const lane = token(styleOf(rail), '--right-stack-max-height', NaN);
-    const chrome =
-      token(stripStyle, 'row-gap', 0) * Math.max(0, tabs.length - 1) +
-      token(stripStyle, 'padding-top', 0) +
-      token(stripStyle, 'padding-bottom', 0);
-    const overhead = (column.offsetHeight || 0) - (strip.offsetHeight || 0);
-    const room = lane - overhead - chrome;
-    const entries = tabs.map((tab) => {
-      const tabStyle = styleOf(tab);
-      return {
-        tab,
-        natural:
-          (tab.firstElementChild?.offsetHeight || 0) +
-          token(tabStyle, 'padding-top', 0) +
-          token(tabStyle, 'padding-bottom', 0),
-      };
-    });
-    const natural = entries.reduce((sum, entry) => sum + entry.natural, 0);
-    if (
-      !(room > 0) ||
-      natural <= room + 0.5 ||
-      entries.some((entry) => !(entry.natural > 0))
-    ) {
-      for (const entry of entries) setHeight(entry.tab, '');
-      return;
-    }
-    let remaining = room;
-    let count = entries.length;
-    for (const entry of [...entries].sort((a, b) => a.natural - b.natural)) {
-      const share = remaining / count;
-      if (entry.natural <= share) {
-        remaining -= entry.natural;
-        count -= 1;
-        setHeight(entry.tab, '');
-      } else {
-        setHeight(entry.tab, `${Math.round(share * 100) / 100}px`);
-      }
-    }
-  }
-
   /**
    * Publish the join: the open tab's bottom on the rail (--rail-tab-bottom,
    * rail pixels; the stylesheet sets the panel's minimum height from it) and
@@ -442,10 +385,8 @@ export function createRightRailTabs({
     // would drop keyboard focus on every class change.
     let cursor = strip.firstElementChild;
     let open = null;
-    const tabs = [];
     for (const panel of panels) {
       const { tab, label } = entryFor(panel);
-      tabs.push(tab);
       if (tab !== cursor) strip.insertBefore(tab, cursor);
       else cursor = cursor.nextElementSibling;
       const collapsed = panel.classList.contains('collapsed');
@@ -464,7 +405,6 @@ export function createRightRailTabs({
       entry.tab.remove();
       entries.delete(id);
     }
-    fitTabs(tabs);
     placeJoin(open?.tab, open?.panel);
   }
 
@@ -490,19 +430,6 @@ export function createRightRailTabs({
   strip.addEventListener('click', onClick);
   strip.addEventListener('keydown', onKeydown);
   observer?.observe(rail, { childList: true });
-  // Tabs holding shared heights do not resize with the lane: watch the rail's
-  // style for a new lane. The join's own writes to that style change no lane.
-  let seenLane = rail.style?.getPropertyValue?.('--right-stack-max-height');
-  const laneObserver =
-    typeof Observer === 'function'
-      ? new Observer(() => {
-          const lane = rail.style.getPropertyValue('--right-stack-max-height');
-          if (lane === seenLane) return;
-          seenLane = lane;
-          sync();
-        })
-      : null;
-  laneObserver?.observe(rail, { attributes: true, attributeFilter: ['style'] });
   rail.prepend(column);
   rail.append(join);
   sync();
@@ -517,7 +444,6 @@ export function createRightRailTabs({
       if (destroyed) return;
       destroyed = true;
       observer?.disconnect();
-      laneObserver?.disconnect();
       resizer?.disconnect();
       strip.removeEventListener('click', onClick);
       strip.removeEventListener('keydown', onKeydown);

src/ui/rightRailTabs.test.mjs

@@ -504,46 +504,6 @@ test('a rail panel hidden at mount gets its tab when its owner unhides it, from
   tabs.destroy();
 });
 
-test('a strip shorter than its tabs is shared: short labels keep their height, the long ones get equal heights', () => {
-  const f = fixture();
-  f.doc.defaultView = {
-    getComputedStyle: (el) => ({
-      getPropertyValue: (name) => el.style.props.get(name) ?? '',
-    }),
-  };
-  f.strip.style.setProperty('row-gap', '4px');
-  const labels = {
-    'pp-toggles': 49,
-    'cctv-panel': 28,
-    'global-context-panel': 49,
-  };
-  for (const [id, height] of Object.entries(labels)) {
-    const tab = f.tabFor(id);
-    tab.style.setProperty('padding-top', '6px');
-    tab.style.setProperty('padding-bottom', '6px');
-    tab.children[0].offsetHeight = height;
-  }
-  // The column holds the strip (160px) and a 40px tool slot; the lane is 200.
-  Object.assign(f.tabs.column, { offsetHeight: 200 });
-  Object.assign(f.strip, { offsetHeight: 160 });
-  f.rail.style.setProperty('--right-stack-max-height', '200px');
-  f.tabs.sync();
-  // Room 152 for naturals 61, 40, 61: CCTV keeps 40, the other two share 112.
-  assert.equal(f.tabFor('cctv-panel').style.props.has('height'), false);
-  assert.equal(f.tabFor('pp-toggles').style.props.get('height'), '56px');
-  assert.equal(
-    f.tabFor('global-context-panel').style.props.get('height'),
-    '56px',
-  );
-  f.rail.style.setProperty('--right-stack-max-height', '400px');
-  f.tabs.sync();
-  assert.equal(
-    f.strip.children.some((tab) => tab.style.props.has('height')),
-    false,
-    'a lane with room gives every tab its natural height back',
-  );
-});
-
 test('a tab shrunk by a short strip joins with corners no larger than half its height', () => {
   const f = fixture();
   // The strip's offsets are relative to its column, the column's to the rail.
@@ -639,18 +599,25 @@ test('the stylesheet keeps phones on border-box, folds the strip into narrow des
   // The tabs give way to a short strip and clip their labels at the end.
   assert.match(
     css,
-    /\.right-rail-tab \{[^}]*flex: 0 1 auto;[^}]*align-items: flex-start;[^}]*min-height: 0;[^}]*overflow: hidden;[^}]*mask-image: linear-gradient\(to bottom, #000 calc\(100% - 6px\), transparent\);/,
+    /\.right-rail-tab \{[^}]*flex: 1 1 0%;[^}]*align-items: flex-start;[^}]*min-height: 0;[^}]*max-height: max-content;[^}]*overflow: hidden;[^}]*mask-image: linear-gradient\(to bottom, #000 calc\(100% - 6px\), transparent\);/,
+  );
+  // The column stands the lane's full height so the strip's size is definite
+  // and a zero basis shares it; only the strip and the slot take the pointer.
+  assert.match(
+    css,
+    /#right-rail-column \{[^}]*height: var\(--right-stack-max-height, 70vh\);[^}]*pointer-events: none;/,
+  );
+  assert.match(css, /#right-rail-tabs \{[^}]*pointer-events: auto;/);
+  assert.match(css, /#right-rail-tools \{[^}]*pointer-events: auto;/);
+  assert.doesNotMatch(
+    css,
+    /#right-rail-column \{[^}]*max-height: var\(--right-stack-max-height/,
   );
   // The rail layout pass measures the column's natural height to climb above
   // its baseline before the strip has to scroll.
   assert.match(
     css,
-    /#right-context-rail\[data-rail-measuring\] > #right-rail-column \{\s*max-height: none !important;/,
-  );
-  assert.match(
-    css,
-    /#right-context-rail\[data-rail-measuring\] #right-rail-tabs > \.right-rail-tab \{\s*height: auto !important;/,
-    'shared tab heights are lifted for the measurement, or the climb would see a column that already fits',
+    /#right-context-rail\[data-rail-measuring\] > #right-rail-column \{\s*height: auto !important;/,
   );
   assert.match(
     css,

src/ui/styles/right-rail-tabs.css

@@ -58,7 +58,9 @@
 }
 
 /* The strip's column: the tablist, then a tool slot that holds the compact
-   Radio launcher on desktop (radioPanelPlacement.js). */
+   Radio launcher on desktop (radioPanelPlacement.js). It stands the lane's
+   full height, which makes the strip's size definite so its tabs can share
+   it (see .right-rail-tab); only the strip and the slot take the pointer. */
 #right-rail-column {
   position: absolute;
   top: 0;
@@ -68,18 +70,13 @@
   flex-direction: column;
   box-sizing: border-box;
   width: var(--right-rail-tabs-inset);
-  max-height: var(--right-stack-max-height, 70vh);
-  pointer-events: auto;
+  height: var(--right-stack-max-height, 70vh);
+  pointer-events: none;
 }
 
-/* The layout pass reads the column's natural height with the lane clamp and
-   the shared tab heights lifted, to climb when the lane is too short for it
-   (rightPanelRail.js). */
+/* The layout pass reads the column's natural height, to climb when the lane
+   is too short for it (rightPanelRail.js). */
 #right-context-rail[data-rail-measuring] > #right-rail-column {
-  max-height: none !important;
-}
-
-#right-context-rail[data-rail-measuring] #right-rail-tabs > .right-rail-tab {
   height: auto !important;
 }
 
@@ -96,6 +93,7 @@
   /* The rail climbs above its baseline to fit the column (rightPanelRail.js);
      a lane still shorter shrinks the tabs. The strip never scrolls. */
   overflow: hidden;
+  pointer-events: auto;
 }
 
 #right-rail-tools {
@@ -106,6 +104,7 @@
   box-sizing: border-box;
   margin-top: 6px;
   padding-left: var(--right-rail-tabs-gap);
+  pointer-events: auto;
 }
 
 #right-rail-tools:empty {
@@ -174,18 +173,22 @@ body:not(.mobile-shell)
   outline-offset: -3px;
 }
 
-/* A short strip is shared between the tabs (rightRailTabs.js): the long
-   labels end up the same height. A tab clips its label at the end, faded over
-   the bottom padding. */
+/* A short strip is shared between the tabs: a zero basis hands the strip's
+   height out in equal parts, a tab's own length caps its part, and what a
+   short label leaves goes to the longer ones, which end up the same height. A
+   strip with room gives every tab its own length. The basis is 0%, not 0:
+   the strip's content size, which sizes the column, comes from the labels. A
+   tab clips its label at the end, faded over the bottom padding. */
 .right-rail-tab {
   position: relative;
   display: flex;
-  flex: 0 1 auto;
+  flex: 1 1 0%;
   align-items: flex-start;
   justify-content: center;
   box-sizing: border-box;
   width: var(--right-rail-tabs-width);
   min-height: 0;
+  max-height: max-content;
   padding: 6px 0;
   overflow: hidden;
   mask-image: linear-gradient(to bottom, #000 calc(100% - 6px), transparent);