Followup to 999e2ff5
Switching between the topic timeline and the progress bar was buggy when
resizing the composer. The root of the problem is that we can't know
the height of the timeline once it's hidden from view.
This uses a magic number for the calucation, which in this case is
necessary. Additionally, the calculation now takes place when
the resizing of the composer ends (previously, it was triggered when
dragging was started, which caused issues when resizing slowly).
f782c3019c