[obaseyi2]

Fixes #762.

Ordinary tab presses suppress pointer propagation and compatibility mouse events, so ancestor listeners cannot dismiss an outside-click popup.

Allow ordinary pointer presses/releases to propagate and prevent mouse focus changes on mousedown. Preserve touch compatibility-event cancellation and active-drag release suppression. Changes are in tabbar.ts and its tests.

Reproduction

1. Attach a TabBar beside a focused input and an open popup.
2. Register an ancestor mousedown listener that hides the popup.
3. Click a tab using a real mouse event.

Before: the listener does not dismiss the popup. After: it dismisses the popup while the input retains focus. Ancestors receive pointerdown, mousedown, pointerup, mouseup and click.

Validation

• Widget suite: 980 tests passed in each of Chromium and Firefox.
• Original source failed two new regressions; the patch passed them.
• Separate native-click checks in both browsers verified event delivery, popup dismissal and retained input focus.
• Source build, changed-file lint/format checks and diff whitespace checks passed.

WebKit and full JupyterLab were not tested. Cancelling pointerdown suppresses compatibility mousedown; moving focus prevention to mousedown is necessary for mouse-based outside-click listeners.