To repro:
- stack 3 dock widgets vertically
- close the top one
- close the top one
The remaining one should now fill the whole window but it wasn't.
Culprit was the code that makes sure that when a dock is closed that
the adjacent ones get to share their space
Silenced a false-positive warning about anchor visibility.
It was invisible because we're within a main window show()
It will be checked by the unit-tests and fuzzer anyway. No need to
do it in production.
