How to control focus in a MDI application

Problem/Question/Abstract:

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:

procedure TMainForm.ComboBox1Click(Sender: TObject);
begin
  { ... other actions }
  if Assigned(ActiveMDIChild) then
    with ActiveMDIChild do
      sendmessage(handle, WM_MDIACTIVATE, 0, handle);
end;