ok so for anyone else who's dealing with this headache, here's what finally solved my issue:
1: Don't use this extension at all, it doesn't give you the control you need and is actually just a lot more complicated than the solution below. Also it does weird things to the scaling and doesn't consistently work.
2: Make a full screen Modal container with a transparent background, and within it, the modal box that contains whatever you want to pop up on screen, can be anything. Then make sure the modal box is aligned to the center of the modal Container. position the modal Container under the main layout where your app activity is.
3: Use the extension below:
4: after hiding the System UI Bar on screen init, set the width and height of the modal container to the dimensions of the actual screen (50DP more than what the screen width will return). You can also get the actual width from an element that is set to fill the screen component. For example, my screen width block returns 775 when the width of an element inside of it returns 825. Do this for height to because some tablets will move the System UI to the bottom of the screen in Landscape mode.
5: Then, in the same screen init block, use the setMargin method to give the modal Container a negative top margin. this should cause it to become overlayed over the components above it.
6: As default, set the modal Container to invisible. Then, when you want it to appear, just set it visible. Will be centered every time, no matter if the System UI is there or not.
I also added some in and out animations with this extension to make the open and close look a bit smoother:
In actuality this is actually just a super simple way of doing this but if you want to see a working example just lmk.