…so you don’t care if it overlays the window? You just want it to fill the bottom of the screen?
If so, you could get the dimensions of the root container and scale the window size however you want. As long as you set the size and location of the popup window in the same script that opens it, the sizing and locating will take place before the popup is painted, and the launch will look as intended.
Here’s a few examples that should be useful for what you’re wanting to do:
• Popup Window and Window Size and Location Problem
• Move Popup Window within bounds of Parent Window
• other examples