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Editorial Team
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Asked: 2026-05-12T08:28:09+00:00

As an extension of this question : TForm.OnResize is sometimes fired before a form

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As an extension of this question:

TForm.OnResize is sometimes fired before a form is first shown, but not always. For example, if BorderStyle is either bsDialog or bsNone, then OnResize will not fire. For all other BorderStyle values (and with all other properties at their defaults), OnResize does fire.

Are there other things that affect whether OnResize will fire before the form is shown? For example, other properties, or combinations of properties, that can affect this?

The OnResize event is a result of the ShowWindow API function sending a WM_SIZE message to the window. That bears repeating: the message is coming from Windows, not from Delphi. It’s a Windows function (ShowWindow) that’s (sometimes) sending the message that triggers the event — so the VCL source code is not really helpful in this case.

Bonus points for definitive answers based on documented ShowWindow / WM_SIZE behavior, e.g. references to MSDN documentation or Petzold books.

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1 Answer

  1. Editorial Team

    Maybe it even depend on user’s display settings or desktop theme or Windows version. If OnResize were giving me problems like this, I would build my program to always expect it and handle it in any situation, no matter what I think to be the cause.

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