You can use 'Blend Mode'. To the demo, I've created a new layer, set it to 'Force own texture' = Yes, and moved 'content' and 'window' to that layer. For 'content' set the Blend mode to 'Source in' - it will disappear. Make a new Sprite ('windowClip') that is just a square that is slightly smaller than 'window' and center it in 'window'. Now right-click 'windowClip' and 'Z Order' -> 'Send to bottom of layer. 'content' will now be visible where it overlaps 'windowClip'.
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