Bug 1777902 don't change the size of system fonts when ui.textScaleFactor is set r=emilio

Depends on D150930

Differential Revision: https://phabricator.services.mozilla.com/D151111

modules/libpref/init/StaticPrefList.yaml

@@ -7654,6 +7654,8 @@
 # on Mac). If browser.display.os-zoom-behavior == 1, then a positive value
 # will be multiplied by the text scale factor; otherwise a positive value is
 # used as-is. This controls the size of a CSS "px" at 100% full-zoom.
+# The size of system fonts is also changed in proportion with the change in
+# "px" sizes. Use "ui.textScaleFactor" instead to only change the size of "px".
 # This is only used for windows on the screen, not for printing.
 - name: layout.css.devPixelsPerPx
   type: AtomicFloat

widget/gtk/nsLookAndFeel.cpp

@@ -1094,8 +1094,11 @@ bool nsLookAndFeel::PerThemeData::GetFont(FontID aID, nsString& aFontName,
       break;
   }
 
-  // Convert gdk pixels to CSS pixels.
-  aFontStyle.size /= gfxPlatformGtk::GetFontScaleFactor();
+  // Convert GDK pixels to CSS pixels.
+  // When "layout.css.devPixelsPerPx" > 0, this is not a direct conversion.
+  // The difference produces a scaling of system fonts in proportion with
+  // other scaling from the change in CSS pixel sizes.
+  aFontStyle.size /= LookAndFeel::GetTextScaleFactor();
   return true;
 }