https://bugs.webkit.org/show_bug.cgi?id=215643
<rdar://problem/67430461>
Reviewed by Alan Bujtas.
Source/WebCore:
The CSS spec says that certain characters have to be invisible. Previously, we were doing this
by just treating these characters as if they were the zero width space character. However, this
is wrong for 2 reasons:
1. If the primary font supports the desired character but doesn't support the zero width space
character, we'll fallback past it and trigger a download for secondary fonts until
we find one that supports the ZWS character (which the page didn't even ask for!)
2. Shaping is Turing-complete, and expects that the correct glyphIDs are fed into the shaper.
We can't just arbitrarily swap out glyphIDs before shaping just because we feel like it.
Instead, the solution is to handle these invisible characters after font fallback, and after
shaping. This patch does it just like we handle adding letter-spacing, word-spacing, and
justification after shaping - by iterating over the glyphs output by the shaper and seeing if
any need to be adjusted.
This patch also includes a fix for the situation where the glyph buffer might be left in an
inconsistent state, where the array of glyphs is shorter than the array of characters -
this new codepath added in this patch revealed this existing problem, and fixes it here too.
Test: fast/text/zwj-ligature.html
* platform/graphics/Font.cpp:
(WebCore::createAndFillGlyphPage):
(WebCore::overrideControlCharacters): Deleted.
* platform/graphics/GlyphBuffer.h:
(WebCore::GlyphBuffer::glyphAt const):
(WebCore::GlyphBuffer::deleteGlyphWithoutAffectingSize):
* platform/graphics/WidthIterator.cpp:
(WebCore::WidthIterator::applyCSSVisibilityRules):
(WebCore::WidthIterator::advance):
* platform/graphics/WidthIterator.h:
Source/WTF:
* wtf/unicode/CharacterNames.h:
LayoutTests:
* fast/text/resources/Ahem-GSUB-ligatures.ttf: Added.
* fast/text/zwj-ligature-expected.html: Added.
* fast/text/zwj-ligature.html: Added.
* platform/mac/fast/text/format-control-expected.txt: Progression! A new ligature is formed that wasn't formed before.
Also, this ligature is formed in both Chrome and Firefox, so we match them now.
* platform/win/TestExpectations:
Canonical link: https://commits.webkit.org/240804@main
git-svn-id: https://svn.webkit.org/repository/webkit/trunk@281389 268f45cc-cd09-0410-ab3c-d52691b4dbfc
8c589fafa5