fix(chat): 把 .chat-log-viewport 的 grid 轨道换成确定值(滚动冻结候选修法)

`grid-template: minmax(0, 1fr) / minmax(0, 1fr)` → `100% / 100%`。

**这是假设,不是已证实的修复。** 产品拍板放进包里,理由是「免得到时候阻塞上线」。

## 假设

`1fr` 是**不确定轨道**:尺寸是剩余空间的一份,所以轨道要在拥有它的盒子之后的
另一趟里定尺寸,里面那个滚动盒再被拉伸到轨道最终定下的值。`100%` 直接从这个
viewport **已经确定**的内容盒解析(它自己的高度经 `.chat-log-wrap` 的 `flex: 1`
确定,再往上由 `.split` 的 grid 轨道确定),于是这一格第一趟就是对的,之后不再被
修订。

猜的是:滚动节点在轨道尺寸修订落地之前就建好了,而那次修订没有把绘制属性标脏。
**这条因果链没有建立。**

## 已知的缺陷形状(三次真实现场)

天花板被冻在一个早期的内容高度上(实测 91+583=674、824+583=1407、25+583=608 ——
**天花板 + 视口 = 冻结那一刻的内容高度**),程序性写 `scrollTop` 仍然有效,
只有销毁并重建那个布局盒才能解除。

## 怎么判它对不对

**读 `client_chat_scroll_frozen`**:如果带着这条规则的包仍然在报,假设就是错的,
这一行该换成下一个候选需要的写法。这也是为什么本次同时放开了探针的会话级上报
上限 —— 判据要靠事件量,不能报满三条就变瞎。

⚠️ **代价要说清**:如果冻结从此不再出现,我们**分不开「真的修好了」和「只是把
时序推开了」**。这是产品知情后的取舍。

## 等价性

代码注释声称「跨两种书写方向和五种布局状态,计算几何(含 used track sizes)与
`elementFromPoint` 探针逐字段与 `minmax(0, 1fr)` 相同」—— 那是**实测得来的**,
不是假设。`chat-log-viewport-definite-tracks.test.ts` 守的是 CSS 文本契约
(轨道保持确定、不退回 `1fr`/`minmax()`、`0e8bbdaa69` 的 rtl 滚动条沟槽验收
不被破坏、百分比轨道依赖的「祖先必须是确定包含块」这个前提仍然成立)。

**它证明不了修复有效** —— jsdom 不做布局,没有 used track size、没有滚动边界、
没有合成器,假装能量的测试就是假绿。假设的证伪路径是遥测事件,不是这个套件。

apps/web/src/styles/viewer/composio.css

@@ -4395,7 +4395,42 @@ body.entry-resizing { cursor: col-resize; user-select: none; }
      both logical edges with an explicit grid cell while the rail / jump /
      Plan controls remain positioned overlays. */
   display: grid;
-  grid-template: minmax(0, 1fr) / minmax(0, 1fr);
+  /* Definite tracks — `100%`, not `minmax(0, 1fr)`. This is not a style
+     preference; it removes a resolution step.
+
+     `1fr` is an indefinite track: the size is a share of free space, so the
+     track is sized in a later pass than the box that owns it, and the scroll
+     box inside it then has to be stretched into whatever the track settled
+     on. `100%` resolves straight off this viewport's already-definite content
+     box (its own height is definite via `flex: 1` inside `.chat-log-wrap`,
+     which is definite down from the `.split` grid track), so the cell is the
+     right size on the first pass and nothing revises it afterwards.
+
+     HYPOTHESIS, NOT A CONFIRMED FIX. The defect being chased is the chat log
+     going unscrollable while layout stays correct: the compositor keeps a
+     maximum-scroll bound frozen at an early content height (measured live at
+     91+583=674, 824+583=1407, 25+583=608 — bound + viewport = the content
+     height at the instant it froze), programmatic `scrollTop` still works,
+     and only destroying and rebuilding the layout box clears it. The guess is
+     that the scroll node gets built before the revised track size lands and
+     the revision never marks the paint property dirty. That causal chain is
+     NOT established. Treat this as an experiment we shipped, and read the
+     verdict off `client_chat_scroll_frozen` (see
+     `apps/web/src/observability/chat-scroll-freeze.ts`): if that event keeps
+     arriving from builds carrying this rule, the hypothesis is wrong and this
+     line should go back to whatever the next candidate needs.
+
+     Equivalence to the previous writing is measured, not assumed: across both
+     writing directions and five layout states, computed geometry (including
+     the used track sizes) and the `elementFromPoint` probes that
+     `e2e/ui/split-resize-scrollbar-hitbox.test.ts` asserts are field-for-field
+     identical to `minmax(0, 1fr)`.
+
+     Precondition: a percentage track needs a definite containing block. If
+     `.chat-log-wrap` / `.pane` ever stop handing this viewport a definite
+     height, `100%` degrades to `auto` (content-sized) where `1fr` would still
+     fill — so revisit this line if that chain changes. */
+  grid-template: 100% / 100%;
 }
 .chat-log-viewport > .chat-log {
   grid-area: 1 / 1;

apps/web/tests/styles/chat-log-viewport-definite-tracks.test.ts

@@ -0,0 +1,136 @@
+import { readFileSync } from 'node:fs';
+import { describe, expect, it } from 'vitest';
+
+/**
+ * `client_chat_scroll_frozen` — the chat log stops responding to real wheel
+ * input while layout stays correct, because the compositor holds a
+ * maximum-scroll bound frozen at an early content height.
+ *
+ * The shipped candidate fix is to size `.chat-log-viewport`'s grid cell from a
+ * DEFINITE track (`100% / 100%`) instead of an indefinite one
+ * (`minmax(0, 1fr)`), so the scroll box's cell needs no free-space
+ * distribution pass and is never revised after the scroll node is built.
+ *
+ * WHAT THIS FILE CAN AND CANNOT PROVE
+ *
+ * It cannot prove the fix works. jsdom performs no layout: there are no used
+ * track sizes, no scroll bounds and no compositor here, and a test that
+ * pretended otherwise would be a fake green. The falsification path for the
+ * hypothesis itself is the telemetry event, not this suite.
+ *
+ * What it does hold is the CSS text contract, which is exactly what a careless
+ * later edit would break silently:
+ *
+ *   1. the track stays definite (nobody reverts to `1fr` / `minmax()`);
+ *   2. the acceptance of commit 0e8bbdaa69 ("fix(chat): own rtl scrollbar
+ *      gutter", asserted by e2e/ui/split-resize-scrollbar-hitbox.test.ts) is
+ *      preserved — the log still fills the cell to both logical edges and the
+ *      viewport is still the containing block for the absolute overlays;
+ *   3. the precondition a percentage track depends on — a definite containing
+ *      block — is still handed down the `.pane` / `.chat-log-wrap` chain. If
+ *      that chain loses its definite height, `100%` degrades to `auto`
+ *      (content-sized) where `1fr` would still fill, which would be a silent
+ *      layout regression rather than a loud one.
+ */
+
+const composioCss = readFileSync(
+  new URL('../../src/styles/viewer/composio.css', import.meta.url),
+  'utf8',
+);
+const shellCss = readFileSync(new URL('../../src/styles/shell.css', import.meta.url), 'utf8');
+const indexCss = readFileSync(new URL('../../src/index.css', import.meta.url), 'utf8');
+
+function stripComments(css: string): string {
+  return css.replace(/\/\*[\s\S]*?\*\//g, '');
+}
+
+/** Declaration bodies of every rule whose selector list contains `selector` exactly. */
+function cssDeclarations(css: string, selector: string): string[] {
+  const blocks: string[] = [];
+  const rulePattern = /([^{}]+)\{([^}]*)\}/g;
+  const source = stripComments(css);
+  let match: RegExpExecArray | null;
+
+  while ((match = rulePattern.exec(source)) !== null) {
+    const selectors = (match[1] ?? '').split(',').map((item) => item.trim());
+    if (selectors.includes(selector)) blocks.push(match[2] ?? '');
+  }
+
+  return blocks;
+}
+
+/** Every stylesheet `index.css` pulls in, in its real cascade order. */
+function importedStylesheets(): { specifier: string; css: string }[] {
+  const specifiers = [...indexCss.matchAll(/@import\s+'([^']+)'/g)].map((m) => m[1] as string);
+  expect(specifiers.length).toBeGreaterThan(30);
+  return specifiers.map((specifier) => ({
+    specifier,
+    css: readFileSync(new URL(specifier, new URL('../../src/index.css', import.meta.url)), 'utf8'),
+  }));
+}
+
+describe('client_chat_scroll_frozen — chat log viewport sizes its scroll box from a definite track', () => {
+  it('sizes the grid cell with definite tracks, not free-space distribution', () => {
+    const blocks = cssDeclarations(composioCss, '.chat-log-viewport');
+    expect(blocks).toHaveLength(1);
+    const viewport = blocks[0] as string;
+
+    expect(viewport).toMatch(/\bdisplay:\s*grid;/);
+    expect(viewport).toMatch(/\bgrid-template:\s*100%\s*\/\s*100%;/);
+
+    // The whole point of the change: no indefinite track may come back in.
+    // `1fr` is sized from leftover space in a later pass than the box that
+    // owns it, which is the resolution step this rule exists to remove.
+    expect(viewport).not.toMatch(/\bminmax\(/);
+    expect(viewport).not.toMatch(/\d*\.?\d*fr\b/);
+  });
+
+  it('keeps commit 0e8bbdaa69 intact: the log owns both logical edges of the viewport', () => {
+    const viewport = cssDeclarations(composioCss, '.chat-log-viewport')[0] as string;
+
+    // `.chat-message-rail` and `.chat-bottom-float-slot` are absolutely
+    // positioned against this box; dropping `position: relative` re-parents
+    // them to the nearest ancestor and moves the overlays off the panel.
+    expect(viewport).toMatch(/\bposition:\s*relative;/);
+    expect(viewport).toMatch(/\bflex:\s*1;/);
+    expect(viewport).toMatch(/\bmin-height:\s*0;/);
+    expect(viewport).toMatch(/\bmin-width:\s*0;/);
+
+    // The scrollbar gutter is flush with the resize handle only while the log
+    // is stretched across the single cell. e2e/ui/split-resize-scrollbar-hitbox
+    // hit-tests 1px and 3px inside that edge, in LTR and RTL.
+    const log = cssDeclarations(composioCss, '.chat-log-viewport > .chat-log');
+    expect(log).toHaveLength(1);
+    expect(log[0]).toMatch(/\bgrid-area:\s*1\s*\/\s*1;/);
+    expect(log[0]).toMatch(/\bmin-height:\s*0;/);
+    expect(log[0]).toMatch(/\bmin-width:\s*0;/);
+  });
+
+  it('leaves composio.css the sole owner of the viewport, so no later sheet can re-introduce an indefinite track', () => {
+    const owners = importedStylesheets()
+      .filter(({ css }) => stripComments(css).includes('.chat-log-viewport'))
+      .map(({ specifier }) => specifier);
+
+    // A second sheet later in the cascade — routines.css already wins over
+    // chat.css elsewhere on source order alone — could restore `1fr` without
+    // touching this rule at all.
+    expect(owners).toEqual(['./styles/viewer/composio.css']);
+  });
+
+  it('keeps the definite containing block a percentage track resolves against', () => {
+    // `100%` needs a definite containing block. This is the chain that
+    // supplies it: the split's chat track -> .pane -> .chat-log-wrap ->
+    // .chat-log-viewport (`flex: 1`). Track ownership on `.split` itself is
+    // covered by tests/styles/chatpane-grid-transition.test.ts.
+    const pane = cssDeclarations(shellCss, '.pane').join('\n');
+    expect(pane).toMatch(/\bdisplay:\s*flex;/);
+    expect(pane).toMatch(/\bflex-direction:\s*column;/);
+    expect(pane).toMatch(/\bmin-height:\s*0;/);
+
+    const wrap = cssDeclarations(composioCss, '.chat-log-wrap').join('\n');
+    expect(wrap).toMatch(/\bdisplay:\s*flex;/);
+    expect(wrap).toMatch(/\bflex-direction:\s*column;/);
+    expect(wrap).toMatch(/\bflex:\s*1;/);
+    expect(wrap).toMatch(/\bmin-height:\s*0;/);
+  });
+});