TListBox repaint causes other forms to repaint as well

I am experiencing a strange behaviour with a TListBox component, which I use
as an output medium for Write and WriteLn procedures. Whenever an update to
its (TListBox) Items StringList is done, ie a string is added, it causes
other forms in my application to repaint as well. Also other applications
windows which are covering my own app are flickering when the StringList is
updated.

My app is an MDI application and the ListBox is on one of the MDI child
forms. I am using Delphi 4, but Delphi 3 showed the same behaviour (I
think).

Does anybody know the reason for this?

Koen van Dijken