Mouse scrolling should trigger syntax highlighting for newly revealed files

[benvinegar]

Problem

In large multi-file reviews, mouse-wheel / trackpad scrolling can reveal diff sections that stay in the plain-text fallback until some other navigation action happens.

Examples that seem to wake highlighting back up:

• change hunk ([ / ])
• page navigation (Space, PgUp, PgDn)
• other focus-changing navigation paths

The result is that syntax highlighting feels inconsistent depending on how you arrived at a region.

Current implementation notes

A few places look relevant:

• src/ui/components/panes/DiffPane.tsx
  • computes visibleViewportFileIds
  • decides which files get shouldLoadHighlight
• src/ui/diff/useHighlightedDiff.ts
  • only starts async highlighting when shouldLoadHighlight becomes true
• src/ui/components/panes/DiffSection.tsx
  • passes shouldLoadHighlight into PierreDiffView

There is already infrastructure for viewport-aware loading and prefetching, but the wheel/trackpad path does not seem to produce the same highlighting behavior as hunk/page navigation in practice.

There is also existing scroll coverage in test/ui-scroll-regression.test.tsx, so this should be testable at the pane/app level.

Desired behavior

When mouse scrolling reveals a new file or hunk in the review stream, syntax highlighting should begin loading for that newly visible content without requiring an additional keyboard navigation step.

Acceptance criteria

• Mouse-wheel / trackpad scrolling into a new visible file starts highlight loading for that file.
• Highlight behavior is consistent across:
  • mouse scrolling
  • hunk navigation
  • page navigation
• Already-cached highlights still render immediately when revisiting a file.
• Add a regression test that reproduces the mouse-scroll path specifically.

Notes

This is a UX consistency issue more than a pure perf issue: the content is visible, so users expect it to behave the same regardless of which navigation path got them there.