fix(notifications): map the balloon window from the queue, not layout

Deriving the window's visible from column.implicitHeight deadlocked it:
a hidden window stops polishing, Column computes implicitHeight during
polish, so once the column was empty the window never re-mapped and no
later balloon could show, including at startup where the queue arrives
asynchronously after the window is already hidden. The binding now reads
Notify.queue membership, which a property binding re-evaluates whether or
not the window is mapped, so a later notification re-maps it. Expired
entries left in the queue keep a zero-height window mapped, which is
harmless.

0 files changed, 0 insertions, 0 deletions