fix: scope stack comments to connected branches only

[nvandessel]

Summary

• Add ConnectedStack() function in internal/dag/dag.go that computes the subset of branches belonging to the same connected stack as a target branch (walk up Parent chain to trunk, then include all descendants)
• RenderStackComment now filters to only connected branches before rendering, so each PR's comment shows only its own stack
• RenderMergedStackComment receives pre-filtered branches from the caller (sync.go), which computes connected sets before deleting merged branches
• Fixes the bug where two independent stacks (e.g., main -> a1 -> a2 and main -> b1 -> b2) would both appear in every PR's stack comment

Test plan

• Added 7 unit tests for ConnectedStack covering: single stack, two independent stacks, target as leaf/middle/root, branching stacks, unknown target, single branch
• Added integration test TestRenderStackComment_FiltersToConnectedStack verifying that RenderStackComment excludes unrelated stacks
• Added test for merged comment with pre-filtered branches
• All existing tests continue to pass (go test ./...)
• RenderTree (used by frond status) is unchanged and still shows all branches
• Quality gates pass: go build, go vet, gofmt -l

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

This PR fixes a bug where independent stacks (e.g., main → a1 → a2 and main → b1 → b2) would all appear in every PR's stack comment, even though they are unrelated. The fix introduces a ConnectedStack function in the DAG package that computes the subset of branches belonging to the same connected stack as a target branch by walking up the parent chain to trunk and then collecting all descendants. RenderStackComment now filters branches through ConnectedStack before rendering, and the sync flow pre-computes connected sets before state mutation so merged PR comments also show only their connected stack.

• Core algorithm: ConnectedStack walks the parent "spine" to trunk, then collects all descendants of spine nodes via DFS — correctly excluding sibling stacks that share the same trunk
• Two-phase merge handling: Connected sets are captured pre-merge in sync.go, then intersected with post-merge state to produce accurate remaining-stack data
• Well-tested: 9 new unit tests for ConnectedStack covering single/multi-stack, leaf/middle/root targets, branching topologies, and unknown targets, plus integration tests for filtered rendering
• No behavioral changes to RenderTree (used by frond status) which continues to show all branches

Confidence Score: 5/5

• This PR is safe to merge — it's a well-scoped bug fix with thorough test coverage and no risky changes.
• The changes are clean, well-structured, and thoroughly tested. The ConnectedStack algorithm is correct — I traced through all edge cases including independent stacks, branching stacks, spine-only walks, and trunk boundary detection. The two-phase pre/post-merge computation in sync.go correctly handles state mutation ordering. No new dependencies or architectural changes. All existing public APIs maintain backward compatibility.
• No files require special attention.

Important Files Changed

Filename	Overview
internal/dag/dag.go	Adds ConnectedStack function with correct spine-walk + descendant-collection algorithm; integrates filtering into RenderStackComment. Logic is sound and well-tested.
cmd/stack_comments.go	Refactored updateMergedComments to accept pre-filtered connected sets and compute readiness per-stack. Clean signature change with correct per-branch readiness computation.
cmd/sync.go	Pre-computes connected stacks before state mutation, then intersects with post-merge DAG to produce accurate remaining-stack data for merged comments. Well-structured two-phase approach.
internal/dag/dag_test.go	Adds 9 new tests covering ConnectedStack edge cases (single/multi-stack, leaf/middle/root targets, branching, unknown target) plus integration tests for filtered rendering. Good coverage.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["RenderStackComment(trunk, branches, highlight)"] --> B{"highlight != '' ?"}
    B -- Yes --> C["ConnectedStack(branches, highlight)"]
    B -- No --> D["Use all branches"]
    C --> E["Walk up Parent chain to trunk (spine)"]
    E --> F["Collect all descendants of spine nodes (DFS)"]
    F --> G["Filter branches to connected set"]
    G --> H["renderTree(trunk, filtered, ...)"]
    D --> H

    I["sync: runSync"] --> J["Pre-compute connectedSets before merge"]
    J --> K["Process merges: delete branches, reparent children"]
    K --> L["Intersect pre-merge connected sets with post-merge DAG"]
    L --> M["updateMergedComments(ctx, st, mergedData, postConnected)"]
    M --> N["Per merged branch: compute readiness from connected subset"]
    N --> O["RenderMergedStackComment with filtered branches"]
    K --> P["updateStackComments(ctx, st)"]
    P --> A

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_{Last reviewed commit: d7a9de2}