ahh, I see it now. Thankyou for reporting this. Infact this is a regression bug. It is fixed in the upcoming release along with a lot of other enhancements to the editor. I don't have an exact date on the release but it will be very soon.
In the meantime, you can workaround this problem by changing the IconsMode to Icons instead of ImageSprites :
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<cc1:Editor ID="Editor1" runat="server" IconsMode="Icons"...
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In icons mode the problem is not there. I am unable to give you another easy fix for this at this time.
Have a good day,