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

この issue ではファイルやテストが指定されていないため、まず diff ビューの実装と、その仮想化、折りたたまれたセクション、プレビュー、Load diff の各パスを追跡します。提案されたハイブリッド動作を比較し、非表示セクションとレンダリング済みセクションを対象としたネイティブ検索のテストを定義します。大規模な diff のパフォーマンス特性を損なわずに一致箇所を検索できれば完了です。

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.