It should fix both problems in the Shape Editor - although the Weapons page looks now horizontally centered, I have no idea why.

It should fix most problems in the Egg Editor, too.

You can see that I added some "valign" properties set to "start", instead of the default "fill", and one "expand" set to "False" instead of "True", the "valign" of that widget was not enough.

At this point I do not understand why I need those. I have to compare with GTK+2. You may have helped me actually in identifying the pages of the Egg Editor as they are simple direct children of the notebook widget [ the widget with the pads ]. The pages of the Shape Editor are more intricate with some levels of intermediate boxes before you reach the actual widgets that are leaves. I am not sure to be able to conclude from the coincidence that all damaged widgets are deep children of some notebook page.