1. Create new project and set fullscreen mode to exclusive
2. Start as windowed 800x480 window
3. Hold alt and quickly tap <enter> twice
4. Once player is windowed, notice that now it is a bit larger than before
5. Repeat steps 3-4 for couple more times and notice that window now takes up almost whole screen
On 5.5 and 5.6 size doesn't increase - initial resolution is preserved. On 2017.1 and 2017.2 window after returning from fullscreen takes up whole window.