We are not aware of such problem with the RadContextMenu. I tested a sample page using the Grid Context Menu demo
code but I was not able to replicate such issue.
One thing that I noticed is that you set explicitly css width to "100%" for some items. Please remove all logic for adding additional styles and test it again.
In order to investigate this current behavior we would need a sample runnable page/code that replicates the issue.
Also you can try calling the RadContextMenu client-side object repaint() method and see if this will fix the issue.
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