By using Maximize action the client size won't go full, only the full window size.
I want to simply make the client size fullscreen, not the window size, but the clientsize, but by doing this the Y size of the clientsize is a little smaller than what it should be.. why I don't know..
you can test by yourself by making a test game, select window width and heigth in the application properties and change to your current resolution, also select the option to show more content in 'resizing'(not really necessary) and enable caption. Test the game and check the current Client size, it's not what you added in the properties, at last here is not.
Add 1 text object and in events make always set text to: ClientWindow.Width&"x"&ClientWindow.Height