Based on the provided information it would be hard to pinpoint the exact cause of the behavior. It could be due to the custom logic used to resize the grid or the browser or a different reason.
Would you try to remove the custom code that is applied to resize RadGrid and see if the behavior changes?
Also, please test the behavior in different browsers and see if it makes a difference.
In case the issue persists please send us the markup for the grid with the relevant client- and server-side code. This way we can examine the code and also try to reproduce the issue locally.
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