These issues affect glyph rendering (the aspect of individual glyphs).
The same glyph appears with different amount of hinting in the same line of text, causing a very odd and blurry appearance. Some glyphs are fully hinted, some look like they’ve been offset by a fraction of a pixel.
Known affected applications:
Even though applications using webkitgtk seem to be affected, the issue is in Cairo. The issue appeared in version 1.17.4 of the library, and hasn’t been addressed so far.
For now, the solution is to revert (downgrade) to an older version of Cairo.
These issues affect the spacing between glyphs.
The issue appears only when full hinting is enabled. The space between some of the glyphs is larger than expected, making the overall text width larger.
Even though it seems to affect all applications using GTK3, the issue is in Pango. The issue appeared in version 1.44 of the library and was fixed somewhere before version 1.50. Then it reappeared in version 1.50.3.
For now, the solution is to downgrade the library to an older version - either version 1.43 or version 1.50.2, depending on the current/affected version of the library.