thinking content improvements

[justschen]

• performance
• general rendering fix

[Copilot]

Pull request overview

This PR improves the chat “thinking” content rendering path, focusing on reducing layout work during streaming updates and ensuring tool invocations that should be hidden are removed at the right time.

Changes:

• Flush pending tool-removals before first expansion so removed items aren’t materialized from lazyItems.
• Batch scroll dimension updates into a single requestAnimationFrame pass and reuse a measured contentHeight to avoid extra layout reads.
• Remove tool entries when they reach Completed/Cancelled if their presentation is hidden or hiddenAfterComplete.

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatThinkingContentPart.ts:1323

• This adds new behavior to remove tools with presentation === 'hiddenAfterComplete' once they reach Completed/Cancelled, but there’s no corresponding test coverage. Please add a unit test that simulates a tool invocation transitioning to Completed/Cancelled while the thinking part is collapsed and asserts the tool entry is removed (and not rendered later).

						// Remove tools that should be hidden now or after completion.
						if (toolInvocationOrMarkdown.presentation === 'hidden' || toolInvocationOrMarkdown.presentation === 'hiddenAfterComplete') {
							this.pendingRemovals.push({ toolCallId: toolInvocationOrMarkdown.toolCallId, toolLabel: currentToolLabel });
							this.schedulePendingRemovalsFlush();
						}

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