1、Bold fonts become to normal 2、password field become to blank
Every bold fontd get regular wight? Strange.
Perhaps a brokensystem font or a graphics card driver problem?
Where does this happen with password field?
Cant confirm your problems with font or password field on Debian 8x64.
Problem could be with system or DM fonts. Steffie had to change one of her DM fonts to fix broken fonts in the new snapshot, and one of the snapshots borked the fonts of some users on Windows a couple of months back (made them all italic), which they solved by merely re-installing the system font package.
The problem seems to stem from an incompatibility between what Vivaldi wants to show, and what the system or DM has.
In the past i had two issues with Snapshots with fonts on Windows, they were Italic (yes), and it worked as i reinstalled the complete font family.
On Linux i could remeber (but dont know when this was) some issue with Google Droid* font family having broken stems.