Draft: Don't shrink vertically packed dialog tabs to 1 pixel height

Fix a UI bug whereby vertically stacking dialog tabs results in the bottom dialog and the divider above it becoming squashed to just 1 pixel height.

The solution for now is to respect minimum heights so that at least the bottom dialog's tab is visible and the divider is usable. The ultimate source of the bug is that dragged dialogs initially have 1×1 size, so the minimum size must be manually enforced.

In the second commit, a variable is renamed for better code readability.

Ideally, the shrinkage of dialogs should be evenly distributed between them so as to replace the current greedy logic.

The label backportproposed applies only to the first commit.

Fixes #4254 (closed)