Author: Jonas Bilinkevicius

Status: MDI-application. MDI window and one child window. MDI window owns a TPanel 
component, which owns any component which can get focus (TEdit for example). Child 
window owns a TDBGrid component.

Problem: After running this simple test application, the focus is set on child 
window's first focusable component - TDBGrid. After switching focus to TEdit 
component owned by MDI window, there is no more possibility to switch focus back to 
TDBGrid component owned by child window. TDBGrid component is immune to any mouse 
events. Why? It looks like a child window is thinking about still having focus.

Answer:

This is one of the many shortcomings of the Windows MDI framework, it has never 
been designed to cope with controls outside the MDI children that can take the 
focus. You can trick it by sending a WM_MDIACTIVATE message to the active MDI 
child, here demonstrated by an OnClick handler for a combobox on the toolbar:

1   procedure TMainForm.ComboBox1Click(Sender: TObject);
2   begin
3     { ... other actions }
4     if Assigned(ActiveMDIChild) then
5       with ActiveMDIChild do
6         sendmessage(handle, WM_MDIACTIVATE, 0, handle);
7   end;