Rectangular phylogenetic tree doesn't try to take all available space

Scenario to reproduce:

1. Open "data/samples/CLUSTALW/COI.aln".
2. Build a phylogenetic tree.
   Expected state: a tree widget appears with the tree within. The tree is stretched to the whole widget width.
3. Reduce the widget width (if there is no horizontal scrollbar).
   Expected state: a horizontal scrollbar appears.
4. Change the tree layout to "Circular".
5. Zoom in the tree.
6. Change the tree layout to "Rectangular".
   Expected state: the tree is about the same width as it was right after build.
   Current state: the tree width is fit to the widget width. It is too small.