Doing some more investigation on this....here is what I found....I didn't quite understand your answer in relation to the problem....I thought I did, but now I don't.
Display set to 100% zoom level - "Small"
Menu bar shows at 814 pixels wide - look ok in Easy CSS and Preview
Here is again with 125% zoom level - Medium (Default)
Menu bar shows at 908 pixels wide - different as you can see. Wouldn't it be the same?
So at 100% zoom the preview (And resulting menu bar) works fine, but at 125% is doesn't.
I don't recall having this problem in Easy CSS Menu 3 I think...because I have the menu working on a live site right now and my zoom level has always been at 125%.