WebKit Dashboard-tree keyboard focus assertions fail

[BorisTyshkevich]

Problem

tests/e2e/dashboard-tree.spec.js has five deterministic WebKit-only failures in
the keyboard-focus path. The same five failed before and after #530, so this is
not caused by rendering Bool Dashboard variables as checkboxes.

Evidence

• npm run test:e2e before Render Bool dashboard variables as checkboxes #530's Bool-test changes: 536 passed, 5 failed.
• Re-run after Render Bool dashboard variables as checkboxes #530: 539 passed, 5 failed.
• npm run test:e2e -- --project=webkit tests/e2e/dashboard-tree.spec.js:
  40 passed, the same 5 failed.

Affected assertions:

• tests/e2e/dashboard-tree.spec.js:316 — Tab traversal of row actions.
• tests/e2e/dashboard-tree.spec.js:605 — Add-to-dashboard focus restore.
• tests/e2e/dashboard-tree.spec.js:856 and :876 — keyboard invocation of
  the Dashboard title pencil.
• tests/e2e/dashboard-tree.spec.js:960 — keyboard-driven panel deletion.

Each reports that the expected trailing action button is inactive rather than
focused after Tab. Diagnose the WebKit focus order/visibility contract and make
the browser suite green without loosening the keyboard assertions.