fix: keep long conversation content scrollable

Large messages containing reasoning, tool lists, or long replies could use an estimated off-screen height that left later content outside the reachable scroll range. Conversation messages now participate in layout with their actual height while the existing 80-message render window keeps long histories bounded.

Release note: 修复偶发的长会话滚动条提前到底问题；包含较长推理、工具记录或正文时，现在可以直接滚动查看全部已加载内容，无需切换会话刷新。

src/renderer/src/ChatTimeline.test.tsx

@@ -5,6 +5,8 @@ import {
   screen,
   within
 } from '@testing-library/react'
+import { readFileSync } from 'node:fs'
+import { join } from 'node:path'
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import {
   ChatTimeline,
@@ -13,6 +15,10 @@ import {
 
 const markdownRenderProbe = vi.hoisted(() => vi.fn())
 const htmlRenderProbe = vi.hoisted(() => vi.fn())
+const stylesheet = readFileSync(
+  join(process.cwd(), 'src', 'renderer', 'src', 'styles.css'),
+  'utf8'
+).replaceAll('\r\n', '\n')
 
 vi.mock('./MarkdownRenderer', () => ({
   MarkdownRenderer: ({
@@ -60,6 +66,14 @@ describe('ChatTimeline', () => {
     vi.clearAllMocks()
   })
 
+  it('uses actual message heights for the conversation scroll range', () => {
+    const rule = stylesheet.match(/\.message\s*\{([^}]*)\}/u)?.[1]
+
+    expect(rule).toBeDefined()
+    expect(rule).not.toContain('content-visibility')
+    expect(rule).not.toContain('contain-intrinsic')
+  })
+
   it('enables HTML rendering only for completed Agent output', () => {
     const messages: Message[] = [
       {

src/renderer/src/styles.css

@@ -5281,8 +5281,6 @@ button > svg {
 }
 
 .message {
-  content-visibility: auto;
-  contain-intrinsic-block-size: auto 160px;
   display: grid;
   min-width: 0;
   padding: 17px 8px;