fix(mobile): make the phone header tab strip read as live tabs

On a phone every inactive tab rendered transparent: grey 11px text
floating in unmarked gaps, a boxed Alt+N digit in each tab (a phone has
no Alt key), names capped at 50px so a shared `w1-` prefix was most of
what showed, and the tab that did not fit was chopped mid-word against
the connection dot. The strip looked like a row of disabled labels.

Phone block of mobile.css only:
- Every header tab is a chip, filled and bordered from the skin's
  --control-* tokens, name in --text at weight 500. Written
  `:where(.header) .session-tab` so it stays at (0,1,0): the per-colour
  left border still wins, and sidebar layout (where the list leaves the
  header) is untouched.
- The Alt+N digit is hidden in the header; inactive tabs drop their
  empty .tab-actions container, which padded the chip's right side.
- Name cap 50px -> 80px, status dot 4px -> 6px, strip gap 2px -> 6px.
- Scroll-driven edge fade: a mask on the strip whose widths follow its
  own inline scroll timeline (registered @property lengths), so the
  clipped tab dissolves into the edge. No JS; a strip that does not
  overflow gets no mask, and browsers without scroll timelines keep the
  old hard edge.

The tap-zone arithmetic comment is updated for the numberless phone
tabs and the bigger dot (the required reserve drops from 38px to 36px;
the 44px min-width stays). test/mobile-tab-strip-chips.test.ts pins the
(0,1,0) selector, the top-level @property registration and the
timeline-after-shorthand order, each of which fails silently otherwise.
test/mobile/tabs.test.ts follows the new name cap.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/web/public/mobile.css

@@ -332,6 +332,39 @@ html.mobile-init .file-browser-panel {
   }
 }
 
+/* Edge fade for the phone's header tab strip (used in the block below).
+   Registered so the keyframes can interpolate them as lengths; @property is
+   only valid at the top level, hence out here. */
+@property --tab-strip-fade-start {
+  syntax: '<length>';
+  inherits: false;
+  initial-value: 0px;
+}
+
+@property --tab-strip-fade-end {
+  syntax: '<length>';
+  inherits: false;
+  initial-value: 0px;
+}
+
+/* Driven by the strip's own scroll position, not by time: at the start only the
+   end edge fades, at the end only the start edge, and in between both. */
+@keyframes tab-strip-edge-fade {
+  0% {
+    --tab-strip-fade-start: 0px;
+    --tab-strip-fade-end: 28px;
+  }
+  10%,
+  90% {
+    --tab-strip-fade-start: 28px;
+    --tab-strip-fade-end: 28px;
+  }
+  100% {
+    --tab-strip-fade-start: 28px;
+    --tab-strip-fade-end: 0px;
+  }
+}
+
 /* ============================================================================
    Phone Breakpoint (<600px)
    ============================================================================ */
@@ -652,7 +685,7 @@ html.mobile-init .file-browser-panel {
     overscroll-behavior-x: contain;
     scrollbar-width: none;
     max-height: 36px;
-    gap: 2px;
+    gap: 6px;
     padding: 0;
   }
 
@@ -660,6 +693,36 @@ html.mobile-init .file-browser-panel {
     display: none;
   }
 
+  /* Fade the strip's edges while there is more to scroll to, so the tab that
+     does not fit dissolves into the edge instead of being cut mid-word against
+     the connection dot. Scroll-driven, no JS: the timeline is the strip's own
+     inline scroll (keyframes + registered properties above this block). A strip
+     that does not overflow has an INACTIVE timeline, so the animation applies
+     nothing and both widths stay at their registered 0px, which is no mask at
+     all. Browsers without scroll timelines skip the block and keep the hard
+     edge. Header only: in sidebar layout the same list scrolls vertically. */
+  @supports (animation-timeline: scroll()) {
+    .header .session-tabs {
+      -webkit-mask-image: linear-gradient(
+        to right,
+        transparent,
+        #000 var(--tab-strip-fade-start),
+        #000 calc(100% - var(--tab-strip-fade-end)),
+        transparent
+      );
+      mask-image: linear-gradient(
+        to right,
+        transparent,
+        #000 var(--tab-strip-fade-start),
+        #000 calc(100% - var(--tab-strip-fade-end)),
+        transparent
+      );
+      /* The shorthand resets animation-timeline, so the timeline comes after. */
+      animation: tab-strip-edge-fade linear both;
+      animation-timeline: scroll(self inline);
+    }
+  }
+
   /* Smaller tabs for mobile */
   .session-tab {
     flex-shrink: 0;
@@ -671,10 +734,46 @@ html.mobile-init .file-browser-panel {
     border-radius: 4px;
   }
 
-  /* Smaller status indicator on mobile */
+  /* Every tab in the header strip is a chip, not only the active one. Left
+     transparent, the strip read as a row of disabled labels: grey 11px text
+     floating in unmarked gaps, with nothing saying "tap me". Fill and border
+     come from the skin's control tokens, so the four light skins (which repaint
+     the header with --glass-bg) get a matching chip with no override block, and
+     the active tab's !important fill and border in styles.css still win.
+     `:where(.header)` keeps this at (0,1,0): the per-colour left border
+     (`.session-tab[data-color="red"]`, (0,2,0)) must still outrank the
+     border-color here, and in sidebar layout the list leaves the header, so
+     its rows are untouched. */
+  :where(.header) .session-tab {
+    border-radius: 8px;
+    background: var(--control-bg-hover);
+    border-color: var(--control-border-hover);
+    color: var(--text);
+  }
+
+  :where(.header) .session-tab .tab-name {
+    font-weight: 500;
+  }
+
+  /* Only the active tab shows its action icons on a phone (see below), so on
+     every other tab the container is empty but still a flex item, and its gap
+     made the chip visibly wider on the right than on the left. */
+  :where(.header) .session-tab:not(.active) .tab-actions {
+    display: none;
+  }
+
+  /* The boxed digit is the Alt+1..9 shortcut hint. A phone has no Alt key, so
+     here it was only a second grey box inside every tab, and 20px of the name's
+     width. Every header tab is therefore numberless on a phone, which is the
+     case the active-tab reserve below is already sized for. */
+  :where(.header) .session-tab .tab-number {
+    display: none;
+  }
+
+  /* Status dot: 6px so an idle green reads at arm's length (4px was a speck). */
   .session-tab .tab-status {
-    width: 4px;
-    height: 4px;
+    width: 6px;
+    height: 6px;
   }
 
   /* The working dot is the one glance-state a phone needs: keep idle tiny, but
@@ -710,9 +809,12 @@ html.mobile-init .file-browser-panel {
     opacity: 0.5;
   }
 
-  /* Truncate tab names more aggressively on mobile */
+  /* Truncate tab names on mobile. 80px, not the old 50px: session names share
+     a `w1-` style prefix, and at 50px "w1-ingest-pipeline" became "w1-inge…"
+     and a clipped tab just "w1-", which says nothing about which session it is.
+     The 20px the hidden tab number gave back pays for most of the difference. */
   .session-tab .tab-name {
-    max-width: 50px;
+    max-width: 80px;
     overflow: hidden;
     text-overflow: ellipsis;
   }
@@ -726,20 +828,19 @@ html.mobile-init .file-browser-panel {
      difference instead, which costs a little strip space on exactly one tab
      and keeps tap-to-switch the majority of it.
 
-     ⚠️ The floor is set by the 10th tab onward, NOT by the numbered tabs you
-     are looking at. `.tab-number` is rendered only for `_tabIdx < 9` (app.js),
-     so tab 10 loses 16px + a 4px gap off its left and its centre sits 10px
-     further right. The centre clears the icons when
+     ⚠️ The floor is set by a NUMBERLESS tab. `.tab-number` is rendered only
+     for `_tabIdx < 9` (app.js), and the header hides it on phones altogether
+     (above), so every phone tab is that case now; a numbered one would sit 10px
+     further left and hide the problem. The centre clears the icons when
 
          reserved > icons + rightEdge - leftRunUp - gap
-                  =    50 +         9 -        17 -   4  = 38px
+                  =    50 +         9 -        19 -   4  = 36px
 
      with icons = gear 32 + close 20 - close's -2px margin, leftRunUp = border 1
-     + padding 8 + status dot 4 + gap 4, and rightEdge = padding 8 + border 1.
-     Hit testing snaps to whole pixels, so 39px still lands on the gear: the
-     practical floor is 40px and 44px keeps 4px of headroom. A NUMBERED tab
-     clears it at 20px, so reasoning from the tabs on screen is exactly what
-     would put the centre back on the gear. Pinned by
+     + padding 8 + status dot 6 + gap 4, and rightEdge = padding 8 + border 1.
+     Hit testing snaps to whole pixels, so a centre half a pixel short still
+     lands on the gear: the practical floor was measured at 40px (with the
+     older 4px dot) and 44px keeps headroom. Pinned by
      test/mobile-tab-tap-zones.test.ts. */
   .session-tab.active .tab-name {
     min-width: 44px;

test/mobile-tab-strip-chips.test.ts

@@ -0,0 +1,166 @@
+/**
+ * @fileoverview The phone header's tab strip must read as live tabs.
+ *
+ * It used to render every inactive tab transparent: grey 11px text floating in
+ * unmarked gaps, a boxed Alt+N digit in each (a phone has no Alt key), names cut
+ * to 50px so a shared `w1-` prefix was most of what showed, and the tab that did
+ * not fit chopped mid-word against the connection dot. On a phone it looked like
+ * a row of disabled labels.
+ *
+ * The fix is four small rules in the phone block of mobile.css, and each has a
+ * way to be silently undone, which is what this file fences:
+ *
+ * - The chip rule is written `:where(.header) .session-tab` so it stays at
+ *   (0,1,0). Written `.header .session-tab` it would be (0,2,0), tie with the
+ *   per-colour `.session-tab[data-color="red"]` left border in styles.css, and
+ *   win on source order (mobile.css loads later): every colour-tagged tab would
+ *   lose its identity stripe.
+ * - The edge fade is scroll-DRIVEN (no JS). Its two widths must be registered
+ *   with @property to interpolate, and @property is only valid at the top
+ *   level: nested inside the phone @media it is dropped, the keyframes stop
+ *   interpolating, and the fade snaps between states instead of following the
+ *   scroll position.
+ * - `animation` is a shorthand that resets `animation-timeline`, so the
+ *   timeline must be declared AFTER it or the fade silently becomes a 0s time
+ *   animation.
+ *
+ * Parsed with postcss because the declarations live in nested at-rules. The
+ * rendered result (chips on dark and light skins, the fade at both scroll ends)
+ * was checked in a browser; this is the cheap regression fence. Port: N/A.
+ */
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+import postcss, { type AtRule, type Declaration, type Rule } from 'postcss';
+import { describe, expect, it } from 'vitest';
+
+const CSS = readFileSync(resolve(import.meta.dirname, '../src/web/public/mobile.css'), 'utf8');
+const ROOT = postcss.parse(CSS);
+const PHONE_QUERY = '(max-width: 599px)';
+
+/** Declarations of the rule matching `selector` inside the phone block (later rules win). */
+function phoneDeclarations(selector: string): Record<string, string> {
+  const found: Record<string, string> = {};
+  ROOT.walkAtRules('media', (atRule) => {
+    if (atRule.params !== PHONE_QUERY) return;
+    atRule.walkRules((rule: Rule) => {
+      if (!rule.selectors.map((s) => s.trim()).includes(selector)) return;
+      rule.walkDecls((decl: Declaration) => {
+        found[decl.prop] = decl.value.trim();
+      });
+    });
+  });
+  return found;
+}
+
+/** The `@property` rule for `name`, wherever it sits. */
+function propertyRule(name: string): AtRule | undefined {
+  let hit: AtRule | undefined;
+  ROOT.walkAtRules('property', (atRule) => {
+    if (atRule.params.trim() === name) hit = atRule;
+  });
+  return hit;
+}
+
+function declsOf(node: AtRule | Rule): Record<string, string> {
+  const out: Record<string, string> = {};
+  node.each((child) => {
+    if (child.type === 'decl') out[child.prop] = child.value.trim();
+  });
+  return out;
+}
+
+describe('phone header tab strip', () => {
+  describe('chips', () => {
+    const chip = phoneDeclarations(':where(.header) .session-tab');
+
+    it('gives every header tab a fill and border from the skin control tokens', () => {
+      // Tokens, not literals: the four light skins repaint the header with
+      // --glass-bg and define their own --control-* values.
+      expect(chip.background).toMatch(/^var\(--control-bg/);
+      expect(chip['border-color']).toMatch(/^var\(--control-border/);
+      expect(chip.color).toBe('var(--text)');
+    });
+
+    it('keeps the chip selector at (0,1,0) so per-colour borders still win', () => {
+      // The lookup above only matches the exact `:where(.header)` spelling, so a
+      // rewrite to `.header .session-tab` leaves it empty and fails here.
+      expect(Object.keys(chip).length).toBeGreaterThan(0);
+      expect(phoneDeclarations('.header .session-tab')).toEqual({});
+    });
+
+    it('hides the Alt+N digit, which a phone has no key for', () => {
+      expect(phoneDeclarations(':where(.header) .session-tab .tab-number').display).toBe('none');
+    });
+
+    it('drops the empty action container on inactive tabs only', () => {
+      // The active tab's gear and close live in .tab-actions, so the rule must
+      // stay scoped to :not(.active).
+      expect(phoneDeclarations(':where(.header) .session-tab:not(.active) .tab-actions').display).toBe('none');
+      expect(phoneDeclarations(':where(.header) .session-tab .tab-actions')).toEqual({});
+    });
+
+    it('leaves enough name to get past a shared w1- prefix', () => {
+      const maxWidth = Number.parseFloat(phoneDeclarations('.session-tab .tab-name')['max-width'] ?? '');
+      expect(maxWidth).toBeGreaterThanOrEqual(72);
+    });
+  });
+
+  describe('scroll-driven edge fade', () => {
+    it('registers both fade widths at the top level, as lengths starting at 0px', () => {
+      for (const name of ['--tab-strip-fade-start', '--tab-strip-fade-end']) {
+        const rule = propertyRule(name);
+        expect(rule, `${name} is not registered`).toBeDefined();
+        // Nested in @media it is invalid and silently ignored.
+        expect(rule!.parent?.type, `${name} must be top level`).toBe('root');
+        const d = declsOf(rule!);
+        expect(d.syntax).toBe("'<length>'");
+        expect(d['initial-value']).toBe('0px');
+      }
+    });
+
+    it('fades only the far edge at the start and only the near edge at the end', () => {
+      let frames: Record<string, Record<string, string>> = {};
+      ROOT.walkAtRules('keyframes', (atRule) => {
+        if (atRule.params.trim() !== 'tab-strip-edge-fade') return;
+        frames = {};
+        atRule.each((node) => {
+          if (node.type !== 'rule') return;
+          for (const sel of node.selectors) frames[sel.trim()] = declsOf(node);
+        });
+      });
+      expect(frames['0%']?.['--tab-strip-fade-start']).toBe('0px');
+      expect(Number.parseFloat(frames['0%']?.['--tab-strip-fade-end'] ?? '0')).toBeGreaterThan(0);
+      expect(frames['100%']?.['--tab-strip-fade-end']).toBe('0px');
+      expect(Number.parseFloat(frames['100%']?.['--tab-strip-fade-start'] ?? '0')).toBeGreaterThan(0);
+    });
+
+    it('masks the header strip behind a scroll-timeline feature check, timeline after the shorthand', () => {
+      let strip: Rule | undefined;
+      ROOT.walkAtRules('media', (media) => {
+        if (media.params !== PHONE_QUERY) return;
+        media.walkAtRules('supports', (supports) => {
+          if (!/animation-timeline:\s*scroll\(\)/.test(supports.params)) return;
+          supports.walkRules((rule) => {
+            if (rule.selectors.map((s) => s.trim()).includes('.header .session-tabs')) strip = rule;
+          });
+        });
+      });
+      expect(strip, 'no @supports-gated .header .session-tabs rule in the phone block').toBeDefined();
+
+      const props: string[] = [];
+      const d: Record<string, string> = {};
+      strip!.each((node) => {
+        if (node.type !== 'decl') return;
+        props.push(node.prop);
+        d[node.prop] = node.value.replace(/\s+/g, ' ').trim();
+      });
+      for (const prop of ['mask-image', '-webkit-mask-image']) {
+        expect(d[prop]).toContain('var(--tab-strip-fade-start)');
+        expect(d[prop]).toContain('var(--tab-strip-fade-end)');
+      }
+      expect(d.animation).toContain('tab-strip-edge-fade');
+      expect(d['animation-timeline']).toBe('scroll(self inline)');
+      expect(props.indexOf('animation-timeline')).toBeGreaterThan(props.indexOf('animation'));
+    });
+  });
+});

test/mobile/tabs.test.ts

@@ -88,9 +88,10 @@ describe('Tab Navigation', () => {
       if (tabNameExists) {
         const maxWidth = await getCSSProperty(page, SELECTORS.TAB_NAME, 'max-width');
         const maxWidthPx = parseFloat(maxWidth);
-        // Should be 50px on mobile
-        expect(maxWidthPx).toBeLessThanOrEqual(60);
-        expect(maxWidthPx).toBeGreaterThan(0);
+        // 80px on phones: wide enough to get past a shared `w1-` prefix,
+        // still short enough that several tabs fit the strip.
+        expect(maxWidthPx).toBeLessThanOrEqual(96);
+        expect(maxWidthPx).toBeGreaterThanOrEqual(72);
       }
     });