It does the same as window.devicePixelRatio. However a bunch of this code is copy-pasted code trying to scale a canvas, but not messing with full zoom is the right thing to do. The full zoom value in the top level browser.xhtml page is always 1 anyways, and WindowsPreviewPerTab looking at the current browser tab's full zoom is just bizarre... Differential Revision: https://phabricator.services.mozilla.com/D138020