Account for the Firefox font size quantization in the text layer

[calixteman]

Firefox rounds the font size set on a canvas 2d context, once divided by the device pixel ratio, to 7 bits of precision. Hence measureText can return a width up to ~1% off the one the spans are laid out with, which made --scale-x slightly wrong in a zoom dependent way. Measure with a size left untouched by that rounding and rescale the returned width, which is linear in the font size.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.20%. Comparing base (a570239) to head (1ad823c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21810   +/-   ##
=======================================
  Coverage   90.20%   90.20%           
=======================================
  Files         264      264           
  Lines       67322    67329    +7     
=======================================
+ Hits        60729    60736    +7     
  Misses       6593     6593           

Flag	Coverage Δ
browsertest	66.31% <91.66%> (+<0.01%)	⬆️
fonttest	8.91% <ø> (ø)
integrationtest	69.42% <100.00%> (+<0.01%)	⬆️
unittest	58.37% <91.66%> (-0.01%)	⬇️
unittestcli	56.92% <0.00%> (-0.01%)	⬇️

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[timvandermeij]

r=me, with passing browser tests. Thanks!

[calixteman]

/botio browsertest

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

Command cmd_browsertest from @calixteman received. Current queue size: 1

Live output at: http://54.193.163.58:8877/e6305afdc9d9f99/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Received

Command cmd_browsertest from @calixteman received. Current queue size: 1

Live output at: http://54.241.84.105:8877/0112093346dfb1d/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Failed

Full output at http://54.241.84.105:8877/0112093346dfb1d/output.txt

Total script time: 18.10 mins

• Regression tests: FAILED

  different ref/snapshot: 75

Image differences available at: http://54.241.84.105:8877/0112093346dfb1d/reftest-analyzer.html#web=eq.log

[moz-tools-bot]

From: Bot.io (Windows)

---

Failed

Full output at http://54.193.163.58:8877/e6305afdc9d9f99/output.txt

Total script time: 23.39 mins

• Regression tests: FAILED

  different ref/snapshot: 72

Image differences available at: http://54.193.163.58:8877/e6305afdc9d9f99/reftest-analyzer.html#web=eq.log

[calixteman]

/botio browsertest

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Received

Command cmd_browsertest from @calixteman received. Current queue size: 0

Live output at: http://54.241.84.105:8877/385709e28bf0a93/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

Command cmd_browsertest from @calixteman received. Current queue size: 0

Live output at: http://54.193.163.58:8877/5fa67fe83411386/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Failed

Full output at http://54.241.84.105:8877/385709e28bf0a93/output.txt

Total script time: 18.12 mins

• Regression tests: FAILED

  different ref/snapshot: 98

Image differences available at: http://54.241.84.105:8877/385709e28bf0a93/reftest-analyzer.html#web=eq.log

[moz-tools-bot]

From: Bot.io (Windows)

---

Failed

Full output at http://54.193.163.58:8877/5fa67fe83411386/output.txt

Total script time: 23.19 mins

• Regression tests: FAILED

  different ref/snapshot: 98

Image differences available at: http://54.193.163.58:8877/5fa67fe83411386/reftest-analyzer.html#web=eq.log