I am afraid that I cannot be of much further help without being able to reproduce and search for the cause of the problem on my side. The only remaining suggestion I can give you with the current information is to disable all kind of customization applied on your side (CSS styles, custom skins, etc) and see whether the root of the problem is not there.
If this does not help, though, please, follow the steps from this blog post
and send us a runnable version of your project in which we can reproduce and examine the problem.
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