The mobile drawer is a modal dialog, but the Tab trap was bound to .root only,
while the backdrop button renders as a sibling OUTSIDE .root — so Tab from the
backdrop (or any out-of-root node) could escape the modal. Move the trap to the
host keydown listener (keydown is composed, so it catches the backdrop too) and
include the backdrop in the focusable cycle; keep initial focus on the first
real control. Adds a regression test for backdrop/outside-root Tab containment.
Benjamin Taylor
225a3fc646