Description

registerTableWindowHandlers reads editor.getRootElement() at call time and returns a no-op cleanup when it is null. With TablePlugin this is fine because the registration runs from a useEffect, which fires after <ContentEditable> has mounted and called editor.setRootElement(...). With TableExtension, the extension's register() runs synchronously inside buildEditor() — before any root element is mounted — so the window-level pointerdown listener that drives table cell drag selection was never attached. The user sees only the browser's native cell selection, which gets clobbered after a couple of cells, producing the "drag selection stops after ~2 cells" symptom reported in the issue.

This PR rewrites registerTableWindowHandlers to use editor.registerRootListener(...) so the pointerdown listener is attached whenever a non-null root element is present and is torn down cleanly when the root element changes or the listener is disposed. No other call sites or behavior change for TablePlugin.

A regression test in LexicalTableExtension.test.ts builds an editor via buildEditorFromExtensions(TableExtension), calls setRootElement after registration, dispatches a pointerdown on a <td>, and asserts that the attached TableObserver.anchorCell is populated — which is only true if the window handler actually ran.

Closes #8491

Test plan

Before

Running the new regression test against main:

× attaches the window pointerdown handler when setRootElement is called after register
  AssertionError: expected null not to be null
    expect(observerAfter.anchorCell).not.toBeNull();

In the Stackblitz reproduction from the issue, dragging across cells in the TableExtension editor stops extending the selection after ~2 cells, while the equivalent TablePlugin editor continues selecting until pointerup.

After

Full unit suite passes (3058 tests, 1 new):

Test Files  118 passed (118)
     Tests  3058 passed | 1 skipped (3059)

Drag selection in the TableExtension reproduction now matches TablePlugin: the selection extends continuously across cells until the pointer is released.