>I have a pop-up form that uses WinAPI transparency to present curved corners. Because of that, the form is top-level and I have it set to always be on top so that it can sit on top of other forms within the app. When switching to another application entirely (e.g. Outlook), the form is still on top.
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>Is there a way I could use event binding to deactivate this form when the user switches to another application?

Did you try making this form invisible in Deactivate method?