A screenshot of 2 Calculators windows side by side. One is focused.
https://cdn.masto.host/floss/media_attachments/files/110/927/034/374/597/155/original/1d598ca7897ca924.png
Only the focused window matters for "you are here".
There are already plenty of hints:
• Window is maximized
• Title/toolbars not dimmed
• You just opened the window
• Backdrop for modal/transient
• Darker shadow
• In-app cursor / focus indicator
• Window focus notification
See the screenshot, looks pretty obvious to me. I seem to recall the design team working on making it even more obvious for 3rd party apps.
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