vscode: sync Builders sidebar selection with the active builder-diff file

[amrmelsayed]

Problem

When reviewing a builder's diff, the file shown in the diff editor and the file selected/highlighted in the Builders sidebar can drift out of sync.

This became visible once cross-file keyboard navigation landed (#1060: codev.diffNextFile / codev.diffPreviousFile, Ctrl+Alt+] / Ctrl+Alt+[). Stepping through files moves the diff editor correctly, but the Builders sidebar keeps whatever row was last clicked highlighted — so the tree no longer tells you "this is the file you're looking at." The same drift happens when clicking a file inside the multi-file View Diff editor: the editor moves, the sidebar selection doesn't follow.

Proposed mechanic

A single, general "reveal active builder-diff file in the Builders view" sync — analogous to the Explorer's Reveal active file / explorer.autoReveal:

• On onDidChangeActiveTextEditor, if the active editor is a tracked builder-diff file (resolve via the diff-inject registry → { builderId, relPath }), locate the matching BuilderFileTreeItem and buildersView.reveal(item, { select: true, expand: true }).
• Gate it so it only reveals tracked builder files (never hijacks selection for unrelated editors), and consider honoring explorer.autoReveal (or a dedicated codev.* setting) so users who dislike auto-reveal can turn it off.

One listener covers every entry point — keyboard navigation (#1060), clicking a file inside the multi-file View Diff editor, and the per-file diff opened from a sidebar click — instead of bolting sync onto each command.

Why this is its own issue (and not part of #1060)

buildersView is created with createTreeView, so reveal() is available — but revealing a file row needs groundwork that #1060 deliberately stayed out of:

1. getParent is builder-row-only today. BuildersProvider.getParent returns parents only for BuilderTreeItem (for the accordion); for BuilderFileTreeItem / BuilderFolderTreeItem it returns undefined. reveal needs the full parent chain: file → folder(s) (tree mode) → builder → group.
2. File/folder rows have no stable id. reveal matches recreated elements by id; file and folder rows need stable, unique ids (e.g. <builderId>::<relPath>).
3. Accordion id-versioning (vscode: accordion shouldn't affect builder area-group headers, and group expansion state should be ephemeral #913). Builder row ids churn via AccordionRowIds; the reconstructed parent chain must use the current version so reveal can match what getChildren returns.
4. Tree-mode folder hierarchy. In file-tree view mode, a file's parent is a folder row from buildFilePathTree; getParent has to reconstruct that hierarchy to find it.

This is real work in a recently-reworked, fiddly view, so it's cleanly separable from the navigation feature.

Acceptance criteria

• Changing the active builder-diff editor (via keyboard nav, multi-file View Diff click, or per-file diff) selects/highlights the matching row in the Builders sidebar, expanding ancestors as needed.
• Works in both Builders view modes (flat list and file-tree), and under both grouping axes (stage / area).
• No selection hijack for non-builder editors (a normal source file or unrelated diff leaves the sidebar selection alone).
• Plays nicely with the accordion (vscode: accordion shouldn't affect builder area-group headers, and group expansion state should be ephemeral #913) — revealing a file expands its builder without fighting the collapse-others behavior.
• Respects an opt-out (honor explorer.autoReveal or add a codev.* toggle).
• No regression to existing Builders tree behavior (accordion, decorations, per-file diff open).

Suggested protocol

PIR — the dev-approval gate is load-bearing here ("looks fine in unit tests, subtly off when you actually walk it across modes"), and the getParent / id-versioning interaction with the accordion deserves a look at the running tree.

Related

• vscode: next-file / previous-file navigation across files in a Codev View Diff session #1060 — cross-file diff navigation (the feature that surfaced this gap).
• vscode: accordion shouldn't affect builder area-group headers, and group expansion state should be ephemeral #913 — Builders-view accordion + row id-versioning (the constraint the parent chain must respect).
• VSCode native precedent: Explorer's Reveal active file + explorer.autoReveal.

Out of scope

• Reverse sync beyond selection (e.g. scrolling the diff to a hunk when a row is clicked — already opens the file).
• Persisting selection across sessions.

[amrmelsayed]

On it! Working on this with the PIR protocol (plan + dev-approval gates before PR).