css line-height revisted [at least zoom and linux]

The mozilla upstream ticket is https://bugzilla.mozilla.org/show_bug.cgi?id=1397994

Following on from legacy/trac#23104 (moved), it seems that when applied on various (preset) zoom levels, that there are differences between Windows and Linux (I do not have any macOS or macOS X machines to test on)

Tor Browser (and RFP in Firefox) actively ignores site specific zoom levels, and new tabs/windows will open at 100% zoom. But that does not stop someone from using zoom, and indeed the setting stays for the current tab when re-used (even when the domain changes - i.e it is a per tab setting in this context). Examples are poorly designed websites, small devices, users with poor eyesight - where the user is effectively forced to zoom (in or out)

Looking at some test results: I used https://ghacksuserjs.github.io/TorZillaPrint/TorZillaPrint.html#useragent - see the css line-height field (and feel free to zoom and refresh) - also see the attachment for some spreadsheet results (png), which is not definitive, but enough to draw some conclusions.

Clearly the mitigation in Windows covered all zoom settings, so was this a design decision? In Linux, it seems as if zoom was only factored in for 50, 100, 150, 200, and 300 (of the preset zoom levels). Is this because of some limitation in Linux?

As a result, so far, at least 8 zoom levels in TBB on Linux are unique and leak the OS as Linux. The 9th zoom level not covered (30%) is not unique in Firefox overall, but is unique on Tor Browser (it is trivial to detect if Tor Browser is being used, so this is in effect a unique value as well)

Note: for Tor Browser, you're not concerned with the Firefox values, I'm just showing them so you can see that outside of 100% zoom, without FP'ing protection, some results are not necessarily OS specific: e.g. FF62+ Windows and Linux are identical at 50, 67, 80, 90, 150, and 240%.