Fix and backport Bugzilla 1873382 and 1929955: Multiply zoom by spoofed pixel ratio; Correct dPR calculation

Backport Patchset

Book-keeping

Bugzilla Issue(s)

• https://bugzilla.mozilla.org/show_bug.cgi?id=1873382 - d15c9aa0
• https://bugzilla.mozilla.org/show_bug.cgi?id=1929955 - 95a08b6d
• https://bugzilla.mozilla.org/show_bug.cgi?id=1954493 - e74cdc0e4cb1fdfc20eb287a850fa3d0fa0ae589
• https://bugzilla.mozilla.org/show_bug.cgi?id=1957617 - d68d1c9346deaff44ea789233bfcb38396d19d82

Target Channels

• Alpha
• Stable
• Legacy

Notes

problem:

• currently breaks spec (lols) and a red flag since dPR (2) !== dpi (96) etc
  • at 100% zoom we report css/matchmedia etc as the zoom value as if dPR was 1

  • "-moz-device-pixel-ratio": 1 "-webkit-min-device-pixel-ratio": 1 "dpcm": 37.79527499999999 "dpi": 96.00000000000003 "dpi_css": 96 "dppx": 1

    • ^ and these all change when we zoom
  • but we report window.devicePixelRatio as 2
    • ^ and it does NOT change when we zoom, it is hardcoded

solution: fairly straightforward

• change our css/mactmedia values to double (since our base spoof is 2)
• multiply window.devicePixelRatio (which is 2) by zoom
• then everything matches at any zoom level

cc: @fkilic @pierov