chat: wrap markdown tables with horizontal scrollbar

[roblourens]

Fixes #265062

Summary

Markdown tables in the chat panel can have many columns with wide content. Previously the table would overflow its container with no way to scroll, and short cell values like 001 would be aggressively squished to a single character wide.

Changes

Horizontal scrolling

• Each rendered <table> is now wrapped in a DomScrollableElement so it scrolls horizontally using the VS Code custom scrollbar.
• The table is placed inside an intermediate <div> first — passing a <table> directly to DomScrollableElement doesn't work because a table always expands to its content width, so clientWidth === scrollWidth and the scrollbar never appears.
• A scanDomNode callback is registered as a layout participant so the scrollbar re-measures on resize.

Column min-widths

• Per-column min-width is set (in ch units) based on the maximum textContent.length of any cell in that column, capped at 3ch. This prevents columns like 001 from being squeezed to one character wide while keeping the layout flexible for wide content.
• Single-character columns are left alone (no min-width applied).
• This is layout-free — only textContent lengths are read, causing no forced reflows.

Header wrapping

• Table <th> cells now use white-space: normal + overflow-wrap: break-word instead of white-space: nowrap, so headers wrap at word boundaries rather than overflowing.

Refactoring

• The wrapping logic is extracted into a new chatMarkdownTableScrolling.ts module.
• Unit tests added in chatMarkdownTableScrolling.test.ts covering DOM structure, scroll wrapper class, layout participants, min-width capping, and multiple-table handling.

(Written by Copilot)

[Copilot]

Pull request overview

Adds horizontal scrolling support for rendered markdown tables in the chat widget (using VS Code’s custom scrollbar) and improves table readability by preventing overly-squeezed short columns and allowing header text to wrap.

Changes:

• Wrap rendered <table> elements in a DomScrollableElement scroll container and re-measure on resize via layout participants.
• Apply per-column min-width (in ch, capped) derived from max cell text length to reduce aggressive column squishing.
• Update chat CSS for the new wrapper and allow <th> wrapping; add unit tests for DOM wrapping + sizing behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/vs/workbench/contrib/chat/test/browser/widget/chatMarkdownTableScrolling.test.ts	Adds unit tests covering wrapper DOM structure, resize participants, and min-width behavior across tables.
src/vs/workbench/contrib/chat/browser/widget/media/chat.css	Styles the new scroll wrapper and adjusts table header wrapping behavior.
src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatMarkdownTableScrolling.ts	Introduces table wrapping + min-width computation logic for rendered markdown tables.
src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatMarkdownContentPart.ts	Integrates the new table-wrapping helper into markdown rendering.