Comment 9 for bug 166970

There's a fundamental question. What to do if in the same document there are physical units (mm) and screen units (px)? Change document look if DPI has changed or stay as it is?

Two size types (actual screen size, screen*200%, screen*300% and actual physical size, physical*200%, physical*300%) is a poor solution. Maybe, the best solution remaining is to make such a thing.

In IS settings: Screen DPI (just for "physical scale") [ 96 ], and checkbox "Take DPI from system".
In zoom menu make two mutually-exclusive options: "Physical scale (depends on screen DPI)" and "Screen scale (1px=1/90in)"

DON'T SIMPLY TAKE SCREEN DPI FROM SYSTEM, let the user override it - the users tend to set the DPI depending on UI font rendering (at least in Windows), and "system inch" isn't necessarily an inch.