I tried to reproduce the described behavior, but to no avail. You can find attached the sample page, which I used for testing. Please compare it with your actual project and check if there is a difference in the setup. Here
you can view the behavior of the RadDock controls on my end.
The RadDocks on your screenshot appear to be customized a bit. I would suggest checking if there are custom CSS styles that modify the default appearance of the dock controls on your page. In case you have tried to change the skin of the RadDocks with a custom one, reviewing the help article Tutorial: Creating a Custom Skin
If you are still unable to resolve the issue at hand, please try modifying the attached sample so that the problem is reproducible or provide a more detailed description of your scenario and reliable steps for recreating the encountered effect.
All the best,
the Telerik team
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