3 hours ago
For something like adding a new transparent window into a PNG, I just grab the Ellipse Select tool, draw the shape where I want the new window, and hit delete if the layer already has alpha. That’s worked for me every time when I need to add another transparent area, especially after checking the layer has an alpha channel (which you already mentioned). If delete doesn’t give you a checkerboard in that spot, it means transparency isn’t set up right, or the selection is on a wrong layer, which has tripped me up a few times before.