For kovidgoyal:
http://ipparis.fr/documents/calibre.jpg
The problem is not limited to Calibre. It makes Bridge and Adobe CS6 very difficult to use for instance, though Photoshop Elements 13 is fine and I read Adobe CC has a way around: an option to apply the 200% which I maintained for the slider Phossler mentions, which is the default recommended by Microsoft. Perhaps those programs with problems inactivate it?
Other pop-ups in Calibre and Kobo Utilities have narrow lines, but less so, and I know them enough to be able to use them.
For Phossler:
I've tried to increase the slider way abode the 200% mentioned above for other applications (the Adobe ones), and it makes no difference to the readability, though it disturbs all the rest of it.