Why is the resulting window's position off by a few pixels than the expected preset position?

That is because Chrome considers the window's drop shadow as being part of the window's size. Unfortunately, there is no way for a Chrome extension to determine the shadow size so that it could be excluded from the new dimensions or coordinates of the window. The only way to overcome this would be to hard-code the shadow size for each operating system (Windows 7/8/10, Mac OS and various Linux distros) and use that. However, this is just a hack and it's not bulletproof, as there are ways for the users to change the window's shadow size or even completely change the way windows look by using themes.For now, you'll just have to account yourself for the shadow's size on your operating system when setting the position / size of the window.

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