I guess you have an NVidia graphics card.
Yes it's a bug. And it's related to NVidias crap support for 2D graphics...
Some other issues with NVidia DirectDraw drivers are related to scaling (the flickering mouse pointer), blt scaling and blt mirror. You can find out more information on the issue on this link:
http://www.gapidraw.com/gf2/index.html
Feel free to try out the NVidia test application on the page. I did this with my GeForce2GTS and even with the latest drivers these same bugs still exists...
You can in most cases avoid the flickering / tooltip repositioning if you do not size your main window (e.g. 2x size or something like that).
/Johan