To reproduce:
1. Open attached project
2. Open scene main. It contains a canvas and a few images. Notice the initial size of the white boxes
3. Undock the game view - the boxes become stretched (bigger)
4. Enter play mode - the whole canvas reduces in size a bit (can be seen as a slight jump) and the buttons return to their normal size. This persists after leaving play mode, so the initial state with the scaled up buttons should never happen

The same happens when docking the window
Regression since 4.6.0b9