Hi Bruno,
Thanks for bringing this issue into our attention.
I was able to reproduce the issue here and I have created a defect report regarding this.We will start working on it soon. You could avoid the behaviour by resizing the main form which contains the both ToolStrip and docking manager.
Here is the link to track the progress of this issue
Defect
#1632 - Painting issue if the form contains ToolStrip
I will inform you with details,once we fixed this issue.
Regards
Joy