Draw a viewport overlay?

Hello! I’m researching this subject. I’ve seen Boxcutter using it (a small border overlay on top of the viewport to indicate you’re in… box cutter mode, I guess) or the slight occlusion you get when entering pose mode.

While the latter might be handled in C, apparently you can achieve such similar things with python, and I was wondering if you guys could orient me towards where to find such things.

Thanks in advance!

In the Blender text editor, see Templates > Python > Operator Modal Draw for an example of how to do this.

Will look into it, thank you!