[Repo Assist] Fix gh stack add: use last active branch index for top-of-stack check - #21

🤖 This is an automated pull request from Repo Assist, an AI assistant.

Summary

Two related bugs in gh stack add's middle-of-stack guard:

Bug 1 — Blocked from adding at last active branch when merged branches sit above it

Before: With trunk → a → b(merged) and on branch a:

✗ can only add branches on top of the stack; run `gh stack top` to switch to "b"

b is already merged — switching to it and adding makes no sense.

After: Allowed. a is the last active branch, so adding on top of it is correct.

Bug 2 — Not blocked when on a merged branch that is the last overall branch

Before: With trunk → a → b(merged) and on branch b:

• No error shown; a new branch would be created from the merged branch's position.

After: Blocked with a message pointing to a (the last active branch).

Root Cause

The guard compared idx against len(s.Branches)-1 (absolute last branch index), which is wrong when the last branch is merged.

Fix

Compute lastActiveIdx — the index of the last non-merged branch — and use it for both the guard condition and the error message.

lastActiveIdx := len(s.Branches) - 1
for i := len(s.Branches) - 1; i >= 0; i-- {
    if !s.Branches[i].IsMerged() {
        lastActiveIdx = i
        break
    }
}

if idx >= 0 && (s.Branches[idx].IsMerged() || idx < lastActiveIdx) {
    cfg.Errorf("can only add branches on top of the stack; run `%s` to switch to %q",
        cfg.ColorCyan("gh stack top"), s.Branches[lastActiveIdx].Branch)
    return ErrInvalidArgs
}

Note on queued branches

IsQueued() is a transient field populated by syncStackPRs, which runAdd does not call. Queued branches are therefore treated the same as active branches in this check. This is documented in a code comment.

Tests Added

• TestAdd_AllowedOnLastActiveBranchWithMergedAbove: verifies bug 1 is fixed
• TestAdd_BlockedOnMergedBranch: verifies bug 2 is fixed; also asserts the error message names the correct last active branch

All existing TestAdd_* tests continue to pass with the new logic (verified by manual trace).

Test Status

Tests could not be executed locally due to Go toolchain version requirements (go.mod requires go 1.25.7) and network restrictions. CI (test.yml) will run the full test suite. The change is a targeted two-line condition replacement with a supporting loop; all edge cases are traced against the existing test scenarios above.

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network:
  allowed:
    - defaults
    - "sum.golang.org"

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