menu bar painting issue for large DPI

Some of the users of my program report a paint problem with the menu bar. It is a standard WTL application using a default menu bar (NOT a command bar, nothing ownerdrawn). Here is how the problem looks like

http://img513.imageshack.us/img513/5576/ezu5383tmp.png

Sometimes it affects only the menu bar, sometimes the toolbar (rebar) below also gets jumbled. 

It is hard to say when this occurs, when you first open the window all is well but after a while it gets into this messy state (after a few paints)

it only happens for people using a large font for their desktop (increased DPI)

any ideas?
thanks
nikos



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