From bug 373002:
If you type // in a Less (LESSCSS) file, Kate incorrectly opens a new region (it should do it in case of multiline comment, /*…*/). If you try to hide that region (using the arrow that's next to line number) it actually hides everything in the file starting from the comment.
Fixing this bug requires removing ' beginRegion="lessComment" ' from line 924 (of less.xml) and incrementing file version number on line 3.
Single line comments are part of Less, as stated here:
http://lesscss.org/features/#features-overview-feature-comments
This diff fixes the mentioned issue for me.