Flake: G14.4 split-pane drag reads a null tab box under load #646

Problem

test/e2e/g14-split.spec.js:51 (@G14.4, "drag a tab onto an edge to show two panes") fails under load with TypeError: Cannot read properties of null (reading 'x') at dragTabToRightEdge (from.x).

Under the forced condition npx playwright test test/e2e/g14-split.spec.js:51 --repeat-each=60 --workers=8:

• main (79af431): 8 failed, 52 passed.
• epic/636-session-history (d03b539): 7 failed, 53 passed.

It also failed once in a full-suite run on d03b539 while a devbox stack run was going on.

Root cause

dragTabToRightEdge calls tab.boundingBox() without waiting for the tab to be laid out. boundingBox() returns null when the element isn't visible at that instant, for example when the tab strip re-renders after the file tab opens.

Fix

Make the wait deterministic: before reading the boxes, wait until the tab and main are visible (await expect(tab).toBeVisible()), and fail with a clear message if a box is still null. This is a test-only change.

Found while working #638.