Merge Line Diff Bar

Pinned source: TortoiseGit 7338078f8ddd924b8cddee35f512f2286072136d, src/TortoiseMerge/LineDiffBar.cpp, BaseView.cpp::ShowDiffLines, and MainFrm.cpp (LineDiff defaults to true).

The two-pane text comparison has a saved Line diff bar checkbox. Below the panes it displays Base above Mine, following the pointer over an aligned line. Caret selection also updates the preview. The strip uses the same font, tab widths and colored source palettes as the panes. It always displays whitespace and inline changes, independent of the main-pane toggles; original endings use the selected shape/icon setting. Word mode and the general inline length bound still apply. Missing-character ticks are drawn without inserting characters. Each preview line follows its pane's horizontal scroll position.

The strip is read-only and does not become first responder or change selection, Undo, text, staging or disk. Mouse exit, invalid rows, alignment rebuild and window retirement clear it. Single-file comparisons suppress it. Three-way merges hide it initially but retain source View/Locator commands that can reveal it; the connected three-way inspection family implements those commands and shares this renderer.

Native verification is included in the existing grouped Merge and LFS/mail receivers. It covers persistence, pointer-to-row mapping with UTF-16 text, forced whitespace/inline rendering, changed CRLF, light/dark drawing and selection/history/file invariants, plus exit/rebuild/retirement guards.

Remaining: physical pointer/keyboard/VoiceOver and minimum-size acceptance, Windows docking/layout fidelity, line-number margin parity and broader inline matching acceptance. Three-way visibility is implemented in the connected family above, rather than inferred complete from its two-file evidence. Screenshots from focused rendering are QA evidence, not whole-window acceptance.