First a TL;DR: Don't worry, I'll figure something out. But for those interested in all the juicy details...

The exact change you see is a difference in the old font (our super-old DejaVu Sans) and the latest version of DejaVu Sans. I suspect differences in the (legacy) kern table, but will have to investigate further.

The reasons for replacing the fonts. We changed from having a single font with 58,000 glyphs to using multiple fonts instead. This makes it easier to maintain, easier to add new languages. Gives us more glyphs to play with (probably over 100,000 now) that act as a fallback stack. So if you replace the main interface font with something new you still get all the characters in all the other fonts too. The change will also allow us to (later) do complex shaping for languages like Hebrew and Arabic, which require specific per-font information.

So one short-term solution while I am figuring this out is to just change the UI font to the old font. Everything will look like it did with no issues.

One complication is that I have been advocating for - and so far failing to get approval for - to get us more accurate glyph placement. Right now we just do it wrong. And that "wrong" also results in our letters looking too scrunched up. Of course fixing that after years of having text too tight will be users complaining of them looking too loose. LOL.

When the placement is done correctly then you would see no change in width between hinting set to "None" (also our default) and when set to "Slight". This is because slight hinting does not make any horizontal changes, only vertical. If you now switch between them you will see what I mean. And note that "Slight" does not scrunch up the "Wi" in the "Window" in the example captures above. So again, another short-term solution is to turn Hinting to "Slight".

I would like to not only fix the placement, but then to also turn on slight hinting by default.

Another complication is that I want to eventually replace our awkward DejaVu with something more modern. Possibly "Roboto", as it has a modern look, is actively maintained, and matches the Noto family by design. Although the difference in design would require us to increase the font size - As in at UI scale 1X you'd want Roboto at 12, rather than our current 11 to have the same width and visual weight. Looks best with slight hinting.

Another complication is that if we eventually move to full shaping (and therefore get things like ligatures), that would also gives a change in kerning too. Right now we are using the legacy, deprecated "kern" table for kerning. With full shaping that necessarily uses the modern "gpos" table instead. Most fonts have both, with kern just acting as an imprecise and incomplete fallback.

Anyway, will figure out something. But with the goal always being to move us forward to better typographical correctness.

An example of the interface using Roboto (using 12 points at 1X scale), with proper placement and with Slight hinting: