GtkMessageDialog: make adaptive
Here is what it currently looks like and how I suggest to make it look when relevant:
Remain the question of when we should trigger that vertical view:
- When the buttons don't fit horizontally?
- Below a hardcoded dialog width?
- Below a hardcoded parent window width?
- Below a hardcoded screen width?
- A combination of these?
Note that this question of "when to trigger a mobile-specific view" is broader than
GtkMessageDialog and likely should be thought out and discussed at some point as it could affect other widgets, like
GtkDialog to know when to auto-maximize itself on mobile, or
GtkHeaderBar to know when to replace the close button by a back button in mobile transient dialogs.