Hi Roman,
Sorry for the inconvenience caused.
This is a known issue and we have a defect report regarding the same.We will address this as soon as possible.
Here is the link to track progress of this fix.
Defect #254 - Docking designer painting issues when the form has a MainMenu
We will inform you, if any workaround has been developed by us to overcome this issue.
Thanks for bringing this issue into our attention.
Regards,
Malarvizhi.