Implement autoscroll in ListView

[roblourens]

Hopefully fix #274428

Do you think this makes sense in the ListView @joaomoreno @rebornix? It doesn't have to be, I just thought it's maybe easier and I wouldn't mind having a way for the copilot debug tree to auto-scroll down too at some point.

[Copilot]

Pull request overview

This PR implements autoscroll functionality directly in the ListView component to address issue #274428, eliminating the need for custom scroll management logic in consumers like ChatWidget.

Key changes:

• Added a new autoscroll option to ListView that automatically keeps the list scrolled to the bottom when content changes (height updates or splices), if the list was already at the bottom
• Removed ChatWidget's custom scroll tracking logic including previousTreeScrollHeight, scrollAnimationFrameDisposable, and the manual scroll-to-bottom logic in onDidChangeTreeContentHeight()
• Added comprehensive test coverage for the autoscroll feature including edge cases

Reviewed changes

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

File	Description
src/vs/base/browser/ui/list/listView.ts	Implements core autoscroll logic with isScrolledToBottom() method and autoscroll checks in updateElementHeight() and _splice() methods; adds autoscroll option to interfaces
src/vs/base/browser/ui/tree/abstractTree.ts	Adds autoscroll option to tree options interface to enable propagation through the tree abstraction layer
src/vs/workbench/contrib/chat/browser/chatWidget.ts	Removes custom scroll logic and enables the new autoscroll option; cleans up scrollAnimationFrameDisposable and previousTreeScrollHeight state
src/vs/base/test/browser/ui/list/listView.test.ts	Adds three comprehensive tests covering autoscroll behavior on element height changes, splices, and non-interference when not at bottom

[Copilot]

The isScrolledToBottom() method doesn't account for rounding errors. Due to rounding, the scrollTop + renderHeight will not exactly match the scrollHeight. This issue was documented in the old ChatWidget implementation that was removed (which used a 2px tolerance), and the same tolerance is still used elsewhere in the codebase.

For reference:

• The old ChatWidget code (removed in this PR) had: this.tree.scrollTop + this.tree.renderHeight >= this.previousTreeScrollHeight - 2
• The debug REPL still uses: tree.scrollTop + tree.renderHeight >= this.previousTreeScrollHeight - 2 (see src/vs/workbench/contrib/debug/browser/repl.ts:691)
• The ChatWidget still uses 2px tolerance for scroll-down button visibility: this.tree.scrollTop >= this.tree.scrollHeight - this.tree.renderHeight - 2 (line 1578)

Consider changing this to: return this.scrollTop + this.renderHeight >= this.scrollHeight - 2; to maintain consistency with existing scroll detection logic in the codebase.

Suggested change

	return this.scrollTop + this.renderHeight >= this.scrollHeight;
	return this.scrollTop + this.renderHeight >= this.scrollHeight - 2;