Why WM_LBUTTONUP message dispear in Delphi? - delphi

Question

The MouseDown and MouseUp will run when I write like this:
procedure TForm1.Edit1MouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
Label1.Caption:='ddddddd';
end;
procedure TForm1.Edit1MouseUp(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
ShowMessage('mouseup');
end;
But when I write like this, the WM_LBUTTONUP dispear and Edit1MouseUp will not run, why?
procedure TForm1.Edit1MouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
ShowMessage('mousedown');
end;
procedure TForm1.Edit1MouseUp(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
ShowMessage('mouseup');
end;

Answer 1

When you call ShowMessage, that shows a modal window. The call to ShowMessage does not return until the modal window closes. The modal window runs its own message loop, and that eats the mouse up message. So, the WM_LBUTTONUP that is in the message queue already, or is about to be placed in the message queue, is actually processed by the message box rather than your Delphi form.
How exactly is that message processed? Well, it depends. If the message was posted before the modal window is shown, then it will be dispatched to the owner window, which is disabled. If it is posted after the modal window is shown, then it may be dispatched to the modal window.
This is one of the reasons why actions are invoked by mouse up rather than mouse down. Perhaps you've not noticed that yet, but try clicking on a button in any common application and note that the response only occurs when the mouse goes up. Indeed if you press the mouse down on a button, move the cursor away from the button, and then release the button, the action does not trigger.
Now try something similar with your second code sample. Press the mouse down in the edit control but don't release it immediately. Note that the result of the mouse going down is that a modal window is now showing. It runs its own message loop and your form is disabled. Now release the mouse button. Clearly the WM_LBUTTONUP message is going to be pulled off the queue by the modal window's message loop.

Answer 2

In your second scenario, the edit control is never posted a WM_LBUTTONUP, hence OnMouseUp event is not fired.
When you call ShowMessage in OnMouseDown, a dialog is launched. This not only releases the mouse capture from the edit control, it also disables the form. With both, the message has nowhere to be posted: not to a window with the capture, and not to a window under the cursor (see documentation).
You can simulate the behavior with this code, without showing a message:
procedure TForm1.Edit1MouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
Assert(GetCapture = Edit1.Handle);
ReleaseCapture;
Enabled := False;
end;
procedure TForm1.Edit1MouseUp(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
ShowMessage('mouseup'); // will not fire.
end;
The moral is: do not use ShowMessage to debug situations involving activation/focus. As it may not be quite evident as in this case, in general do not use it as a debugging tool at all.