fix - stale code block height during incremental chat layout

[xinyi-gong]

Summary

• Add a computeSize(...) override for SourceViewerComposite so code block height is measured synchronously from the current StyledText content.
• Reuse a shared height helper in both measurement and async bounds refresh to keep scrollbar-height handling consistent.
• Prevent incremental chat layout from caching stale code block heights while large code blocks are still waiting for async setBounds(...).

[Copilot]

Pull request overview

This PR improves chat code-block layout stability by ensuring SourceViewerComposite reports an up-to-date height synchronously (via computeSize(...)), reducing the chance that incremental chat layout caches stale code-block heights while async setBounds(...) is pending.

Changes:

• Override computeSize(...) in SourceViewerComposite to measure height directly from the current StyledText content.
• Factor out a shared getSourceViewerHeight(...) helper and reuse it in refreshScrollerLayout() for consistent scrollbar-height handling.

[jdneo]

One small issue I observed:

When tab through the chat view, the highlighted blue line is missing at the bottom of each code block

[xinyi-gong]

One small issue I observed:

When tab through the chat view, the highlighted blue line is missing at the bottom of each code block

The issue was caused by the new computeSize(...) returning the inner StyledText content height directly, while SourceViewerComposite itself is created with SWT.BORDER. That meant the parent layout did not reserve the outer trim/border space, so the bottom focus border could be clipped.

I updated the sizing logic to convert the measured StyledText client size through computeTrim(...) before returning it from SourceViewerComposite.computeSize(...). This keeps the synchronous code block height measurement while preserving enough space for the composite border/focus outline.

[jdneo]

LGTM