I am using Anki on Arch Linux, with the font Noto Sans Tamil for Tamil rendering installed. The issue is that when one types or pastes வ or ப followed by a combining character (i.e. the combinations பா பி பீ பு பூ ப் பை பெ பே பொ போ பௌ வா வி வீ வு வூ வ் வை வெ வே வொ வோ வௌ), the rest of the text field they are entered in becomes glitched and all characters are replaced by boxes. This appears to be specific to Anki as I can’t reproduce this on Librewolf, a Firefox-based browser; i.e. this doesn’t appear to be a Noto problem.
I know pretty much nothing about Tamil characters and fonts, but I can give you some general suggestions for font issues –
Have you looked in the HTML view of the field [click < > above the field] to see what is being stored in the field that gets represented as boxes?
Have you considered whether Unicode normalization is an issue? If that font doesn’t support normalized characters, they would show up as unknown, which is often represented as a “tofu” box or glyph.
There are several ways to get a font to display in Anki, but they might work differently (and show up in different places) depending on how you set it.
A. Setting the “Editing Font” for that field in your note type.
B. Using HTML to set the font field-by-field and note-by-note [not recommended].
C. Setting the font for a {{field replacement}} in your card templates and Styling, so it will show up on your rendered cards.
I’m confused. That fixed it – so isn’t that the fix? If the Editing Font you had before doesn’t support those characters, changing that field to a font that does support those characters is how you fix the field in the Editor.