I was having this problem, and after reading this thread I uninstalled and redownloaded the windows 0.7.56 version from your site (at 1:53 EST), but I am still seeing the error.
It occurs whenever the "Device" button appears in the toolbar and whenever the application window is resized horizontally. The former problem prevents me from transferring books to my device. Panes within the application can be resized, and as long as the toolbar doesn't resize Calibre seems to operate normally.
Thanks!
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