How to remove the top shadow of a sheet

Question

I need to get rid of the shadow on top of the sheet:

I know that they draw it in a special window, but I have no way to get to it. I tried:

• Scrolling through all the windows filtered by the application using CGWindowListCopyWindowInfo, and then viewing them by id using [NSApp windowWithWindowNumber:]. The window is on the list, but when I look at it by id, I go back.

• The swizzling method for [NSWindow init], [NSWindow initWithRect ...], [NSWindow setFrame:], but not from them is called by this magic window.