If this is what I think it is then it's easy to fix.
There are 2 issues that contribute to this problem with Window (no border) showing black screen:
1. When the values in window.am are position 0,0 and size (your display resolution) windows treats it as exclusive fullscreen, but AM still thinks it's windowed.
2. There is a bug that causes the window.am values being overridden by previously selected window mode (fullscreen and fillscreen) this should change only when you adjust in Window mode using a mouse.
I can push this final commit, after you confirm it works when you:
1. Set window mode to No Border.
2. Exit AM
3. Adjust size in window.am to be less than your display resolution
4. Relaunch AM and try playing game