R8a: restore Hide Other Branches across five node kinds

[dovvnloading]

Summary

Restores the "Hide Other Branches" menu action across chat, code, document, thinking and image nodes, replacing a disabled placeholder with a working branch-isolation toggle.

Problem

All five node kinds' menus carried the item disabled, with the tooltip "Branch visibility isn't built yet". The Qt cutover deleted the legacy scene's toggle_branch_visibility (graphlink_scene.py) without porting a successor.

Legacy's own name is misleading: it never removes anything. It dims every node outside the clicked node's branch — its ancestor chain and everything descended from it — down to a low opacity, restoring full opacity on toggle-off. It is a scene-wide toggle: once active, the button reads "Show All Branches" everywhere, and clicking anywhere clears it regardless of which node's menu the second click came from.

Changes

computeDimmedNodeIds (SceneCanvas.tsx) ports the legacy algorithm to this app's uniform (source, target) edge model. Legacy told "chat replied by chat" and "chat owns a code block" apart with different attribute pairs; the modern backend models both as one edge type, so this tells them apart by kind instead — a node's branch anchor is itself if chat-kind, else its nearest parent if that is chat-kind. The active branch is the set of chat nodes reachable by walking chat-to-chat edges only, in both directions; a content node attached to any branch member counts as active via its anchor, not by the walk expanding through it.

Both the ancestor walk and the descendant search are visited-set-guarded — a deliberate hardening legacy's own equivalent walks lack, since SceneDocument.connect() permits creating a cycle with no validation. Dimming is applied via React Flow's per-node style.opacity, the same mechanism this file already uses for the fade-connections feature, at 0.18 — legacy's own constant, ported unchanged.

Dimming is scoped to exactly the five node kinds that carried this menu item in the legacy app. Every other kind, including Conversation (whose own file already documents excluding this feature deliberately), is left untouched.

Process

Built in two stages. The algorithm and all SceneCanvas state were written and hand-verified against 13 graph scenarios — linear chains, sibling forks, deep descendant chains, content nodes on dimmed branches, focusing from a content node itself, orphaned nodes, manually-created cycles — before any other file was touched. Five agents then wired the mechanical menu-button change into the five node views in parallel from one shared contract.

Three independent reviews followed: one devised its own six adversarial graph scenarios blind to the existing test suite and hand-traced them against the implementation before cross-checking against it; one verified cross-file contract correctness; one checked legacy fidelity and test quality. All three came back clean bar one test-coverage gap — a missing menu-closed assertion in one file's active-state test — fixed directly.

That same direct verification pass caught a real lint error outside the reviewed scope: the original self-heal-on-deleted-origin logic called setState synchronously inside a useEffect, the cascading-render anti-pattern react-hooks/set-state-in-effect exists to catch. Replaced with a value derived during render — simpler, and resolves a stale node reference within the same render instead of one render later.

Verification

Check	Result
Backend suite	1033 passed (unaffected — no backend changes)
Frontend tsc --noEmit / ESLint / tests / build	clean / 0 errors / 841 passed (31 new) / succeeds
Live: real 13-node scene with an actual branch fork	confirmed
Live: toggling dims exactly the sibling branch (5 nodes, multiple kinds incl. a thinking node) to opacity 0.18	confirmed
Live: label flips to "Show All Branches"	confirmed
Live: toggling off restores all 13 nodes to opacity 1	confirmed
Live: console	zero errors

Related

This closes out the four-item deferred-menu-item cleanup started with Key Takeaway/Explainer Note (#169) and Open Document View (#170).