Unexpected Application.OnActivate behavior in Delphi 7

Question

I wrote a D7 application to test the behavior of Application.OnActivate.
This is an important part:

procedure TMainForm.FormCreate (Sender: TObject); begin Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.FormCreate - Begin'); Application.OnActivate := AppActivate; Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.FormCreate - End'); end; procedure TMainForm.AppActivate (Sender: TObject); begin Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.AppActivate - Begin'); ShowWidthsHeights (Sender); Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.AppActivate - End'); end; procedure TMainForm.ShowWidthsHeights (Sender: TObject); begin Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.ShowWidthsHeights - Begin'); Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.ShowWidthsHeights - End'); end; 

After starting the application, the contents of Memo1:
1 - MainForm.FormCreate - Start | 2 - MainForm.FormCreate - End
3 - MainForm.AppActivate - Start
4 - MainForm.ShowWidthsHeights - Start | 5 - MainForm.ShowWidthsHeights - End
6 - MainForm.AppActivate - End

It is right.

But if I then click on the shortcut in the taskbar, these 4 lines will be added to Memo1:
7 - MainForm.AppActivate - Start
8 - MainForm.ShowWidthsHeights - Start 9 - MainForm.ShowWidthsHeights - End
10 - MainForm.AppActivate - End

Why is D7 doing this?
My application is not activated, on the contrary, it is disabled!

---

To check if the Application.OnDeactivate event was fired, I added this event handler:

 procedure TMainForm.AppDeactivate(Sender: TObject); begin Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.AppDeactivate - Begin'); Memo1.Lines.Add (IntToStr (Memo1.Lines.Count + 1) + ' - MainForm.AppDeactivate - End'); end; 

and added this statement to TMainForm.FormCreate:

 Application.OnDeactivate := AppDeactivate; 

After starting the application, Memo1 contained the same 6 lines as in the original case, but clicking on the shortcut on the taskbar led to 8 additional lines in Memo1:

 7 - MainForm.AppDeactivate - Begin 8 - MainForm.AppDeactivate - End 9 - MainForm.AppActivate - Begin 10 - MainForm.ShowWidthsHeights - Begin 11 - MainForm.ShowWidthsHeights - End 12 - MainForm.AppActivate - End 13 - MainForm.AppDeactivate - Begin 14 - MainForm.AppDeactivate - End 

So, my application is deactivated, then activated, and then deactivated again!
This is pretty confusing!

Answer 1

As mentioned in David's comment, it reflects the reception of the WM_ACTIVATE message. http://msdn.microsoft.com/en-us/library/windows/desktop/ms632614(v=vs.85).aspx gives the answer:

Dispatched when a window belonging to a different application than the active window is activated. The message is sent to the application, the window of which is activated, and to the application, the window of which is deactivated.

and message parameters are explained below:

WPARAM:

Indicates whether the window is activated or deactivated. This parameter is TRUE if the window is activated; it is FALSE if the window is deactivated.