ScreenshotFrame previously read master.master.slot_var, but
master is the Notebook and master.master is the inner padding Frame
inside App._build, not App. Use winfo_toplevel() so we always reach
the App where slot_var lives.
Reported as: AttributeError: 'Frame' object has no attribute 'slot_var'
at gui.py:401 during run.py startup.
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