editable_test.dart not font-hermetic on web

[yjbanov]

This test is checking for specific size of the Chinese characters, but our ahem.ttf font doesn't have them (according to fontdrop.info). This causes the browser to fallback to a local font, which can be different from one computer to another.

This causes HHH to fail, for example:

https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket.appspot.com/8846622841805053712/+/u/flutter_test_web_tests/stdout

Somehow the C++ engine is immune to this problem. Perhaps it also falls back to some font, but that fallback font is stable across machines.

[GaryQian]

We should add ideographic sample glyphs to cough (flutter/engine#13649) and change tests to depend on that instead of Ahem for ideographic scripts.

[darrenaustin]

Not sure what fixed this, but I was able to turn the test marked with this bug in editable_test.dart with #87700. I will close this, if you feel it is still not properly addressed, please reopen.

[sigmundch]

Unfortunately this is still happening - the Dart/flutter HHH bot went red again once the test was reenabled in 225a43d

See https://ci.chromium.org/ui/p/dart/builders/ci.sandbox/flutter-engine-linux-web_tests/3108/overview

I believe this is hard to repro because many of our local setups have fallback fonts that would work OK.

Maybe there is a way we can force these processes to not look for an alternative font? Could we use an approach, like using FONTCONFIG_PATH or FONTCONFIG_FILE, to ensure we can repro more easily locally?

[LongCatIsLooong]

Somehow the C++ engine is immune to this problem.

Looks like font fallback is disabled in test mode so those characters are always tofu:
https://github.com/flutter/engine/blob/34794638c8b88ea9cabde44e587ece6d0dc56757/lib/ui/text/font_collection.cc#L143
flutter/engine#4661

Looking at the original issue that for this test, it doesn't seem to have to do with the script or the baseline positions? It's just in those fonts the Chinese characters are generally taller than the 0x20 whitespace char we used to use to measure the line height? If that's the case using Ahem defeats the purpose of the test.

[LongCatIsLooong]

Ah nvm SkParagraph reports font metrics instead of glyph metrics for height. So it looks like the original issue is caused by different typefaces being used. I'll update the test and use a child text span with a larger font size instead.