unfortunately there doesn't appear to be any way of detecting this automatically in JS:
https://developer.mozilla.org/en-US/docs/Web/API/Window/devicePixelRatio

the suggestion there is to add a timer that just periodically polls for changes which is a bit ridiculous here.  adding a check to any other common code path would similarly be a lot of waste cycles.

adding a hook to the resize event itself shouldn't be necessary as that already should recalculate things correctly.

so it'll probably stay broken until the Web API improves.  if we could find an uncommon, but semi-periodic event location, to insert such a check, we might be able to mitigate that way.

Hook on newline, but limit to checking once every n seconds?

Regardless, resize is *not* calculating things correctly, or it's using (or being fed) a stale value of DPI or something.  I can move the window to the other display, and then resize the window and it still has the wrong DPI.  Ctrl-0 is the only way I've found to fix things.

not a fan of any timer based approach

googling around, there might be a way to use the scroll event to detect when the font changes.  i'll have to try it on attached displays to see if it helps.  the algo is covered here:
  https://github.com/marcj/css-element-queries/blob/master/src/ResizeSensor.js