Cannot shrink the compose window
When a compose window has long enough lines to wrap, it can be made wider, and the text will rewrap. But it can be made narrower only until a line needs to rewrap; at that point, the spaces act as if they are non-breaking.
This is the same behavior described in gtk#628 (closed) . It is related to setting margins: if the left and right margins in the compose window are removed, the window can be made narrower and the text rewraps as it should.
Merge request gtk!236 (merged) fixes the issue, but has so far not been accepted.