Do you ever needed to display a message using the OnEnter event?
This example shows how to correctly display a message by using the ShowMessage (or an equivalent function) in OnEnter event.

As you have notice, if you call ShowMessage in OnEnter event, mouse will get a bad behavior.
Remember that this problem only occurs when the control executes the OnEnter event after you clicked it with the mouse.

The trick is by using a user windows message.


const
  UM_SHOWMESSAGE = WM_USER + 256;

// Define this procedure to catch our 'ShowMessage' user message
procedure UMShowMessage(var Message: TMessage); message UM_SHOWMESSAGE;

procedure TForm1.UMShowMessage(var Message: TMessage);
begin
  ShowMessage(PChar(Message.lParam));
end;


To activate the message you just need to put this code in OnEnter event:

PostMessage(Handle, UM_SHOWMESSAGE, 0, LPARAM(PChar('This is a message')));