What you would like to achive here is to have "windows inside tab pages". This is not like it supposed to be! It looks like this:
- Windows OS
- Windows of applications (Window class)
- Containers placed on Window (for example: Panel, TabControl!)
- Controls placed on Windows and Containers (for example: Button, but also containers like Panel!)
So when you look on this you see that it's not ok to put Windows class into TabControl!
So what to do?
Create for example UserControl class and move all your controls from Window to this new UserControl. Next place on your Window TabControl nad on one of it's TabPages put this newly created UserControl.
In this way you'll have a good designed UI. Once again: You do not put Window on your TabPage!