[Bug]: "Merge stack" appears late and pushes the stack picker out from under the cursor #25993

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/web

Steps to reproduce

1. Open a stacked PR you can merge. Public demo: GisTypical/t3-stack-demo (PRs 1–5 are a stack, PR 7 is a single PR).
2. Pick another layer from the stack picker (3/5).
3. Click the stack picker again as soon as it shows.
4. Merge stack pops in, pushes the picker left, and the click opens the merge confirmation instead. This repeats on every layer switch.

Expected behavior

Header controls don't move while data loads, and stacked PRs use the same order as single PRs.

Actual behavior

The picker jumps 105 px left about 0.7 s after a layer switch (1280×800). The linked-threads button also swaps sides of the merge action between single and stacked PRs.

Cause, at e923627:

• Merge stack only renders once the stack query is fresh (PullRequestDetailPanel.tsx:1304), but the picker renders from cached data. The row is right-aligned, so the late button pushes the picker.
• PullRequestStackMenu renders Merge stack next to the picker (PullRequestStackMenu.tsx:202) instead of in the Merge slot, which is why the order differs.

Impact

Minor bug or occasional failure

Version or commit

30f4d63 (dev build); the affected files are unchanged on e923627.

Environment

Linux, Chromium, GitHub stacked PRs made with gh stack.

Screenshots, recordings, or supporting files

Single PR	Stacked, loading	Stacked, loaded

Proposed fix

Which would you accept?

1. GitHub's layout: move the stack picker to the left, next to the PR status, and render Merge stack in the Merge slot.
2. Smaller: render Merge stack in the Merge slot, and keep it mounted but disabled while the stack refreshes.

Both give stacked and single PRs one header order and keep Merge stack disabled until the stack is fresh.