chore(web): mermaid followups

[whoisthey]

Addressing items noticed in the while after intro of mermaid diagrams:

• scroll-to behaviour from diagram button could over/undershoot right-panel diagram
• default diagram zoom-fitting behaviour left some overly wide diagrams looking blown out

Resolution:

• use instant scroll to avoid over/undershoot due to display recalc, also matching behaviour with code reference onClick
• make default diagram zoom-fitting keep x/y fit within container, more pleasant default views (see screenshot below)

Summary by CodeRabbit

• Bug Fixes
  • Mermaid diagrams now use consistent width/height “contain” sizing in both panel and fullscreen, preventing horizontal overflow and container layout issues.
  • Panel auto-sizing remains enabled, but the diagram scale is constrained so it can’t grow beyond the available space.
  • When a diagram is revealed, it now jumps into view instantly (instead of smooth scrolling) for more responsive navigation.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 58f10cc0-c972-452a-a53b-db4319efd684

📥 Commits

Reviewing files that changed from the base of the PR and between d73749e and 040c8bb.

📒 Files selected for processing (1)

• CHANGELOG.md

✅ Files skipped from review due to trivial changes (1)

• CHANGELOG.md

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Walkthrough

This PR changes Mermaid diagram fitting in chat threads to use contain-fit scaling in panel and fullscreen modes, and switches diagram reveal scrolling to instant behavior.

Changes

Chat diagram behavior

Layer / File(s)	Summary
Contain-fit diagram scaling packages/web/src/ee/features/chat/components/chatThread/mermaidDiagram.tsx, CHANGELOG.md	computeFit removes the horizontal enlargement path, updates its description, and always applies contain-fit scaling before the auto-height branch; the changelog notes the same behavior change.
Instant reveal scrolling packages/web/src/ee/features/chat/components/chatThread/referencedSourcesListView.tsx	Revealed diagrams now scroll into view with behavior: 'instant' instead of behavior: 'smooth'.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• sourcebot-dev/sourcebot#1035: Changes the same referencedSourcesListView.tsx scrolling path for selected content, so it is directly adjacent to the reveal behavior updated here.
• sourcebot-dev/sourcebot#1036: Also adjusts referencedSourcesListView.tsx scroll handling for diagram/reference reveal positioning.
• sourcebot-dev/sourcebot#1369: Modifies the Mermaid chat-thread diagram behavior in the same area as this PR’s mermaidDiagram.tsx scaling change.

Suggested reviewers

• brendan-kellam

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title is related to the main change set and correctly points to the mermaid follow-up work.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch whoisthey/mermaid-followups

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