Native browser find (`Ctrl/Cmd+F`) does not search the full diff in diff view

Problem

In diff view, native browser find only searches content that is currently mounted in the DOM. Today that means Ctrl/Cmd+F can miss valid matches because parts of the diff are intentionally hidden or not rendered:

• Virtualized file blocks outside the viewport are not mounted.
• Auto-collapsed files hide their diff content.
• Rendered previews replace source diff text for renderable files.
• Large diffs may show a Load diff placeholder instead of line content.
• Omitted context is not present until expanded.

This makes browser find unreliable for reviewing large or mixed-content diffs.

Proposals

• Add a temporary “browser find mode” on Ctrl/Cmd+F that fully renders the diff, expands hidden sections, and forces source view so native browser find can search the whole diff.
• Make the normal diff mode always fully searchable by keeping all source diff text mounted in the DOM, even when sections are collapsed or previewed.
• Build an app-level search experience instead of relying on native browser find.

Recommendation

Use a hybrid approach:

• For small diffs, keep the normal mode fully searchable without a separate mode.
• For large diffs, enter a temporary browser-find mode on Ctrl/Cmd+F that fully renders the diff and forces source text visible.

This keeps the common case simple while avoiding the steady-state performance and UI cost of fully rendering large diffs all the time.

Notes

• Native browser find cannot tell us which hidden section matched, so lazy “expand only the matching file” behavior is not practical.
• The hybrid approach preserves current performance characteristics for large diffs while improving searchability for most day-to-day reviews.

The issue does not name files or tests, so start by tracing the diff view implementation and its virtualization, collapsed-section, preview, and Load diff paths. Compare the proposed hybrid behavior and define tests for native find across hidden and rendered sections; done means matches are searchable without removing the large-diff performance characteristics.