The moveable pane has a slight vertical fin on the back so that it will close against a mirror image fin on the fixed window thus - theoretically - keeping small critters out. It turns out that the fixed window is bowed slightly...just enough to keep the moveable pane from passing without pushing the bow out. Cheap windows are a PITA.
I haven't come up with a fix, may just put in a screw so the window can never again open :)
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