[tor-bugs] #8455 [Firefox Patch Issues]: Fix @font-face font fallback rendering behavior

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#8455: Fix @font-face font fallback rendering behavior
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 Reporter:  mikeperry                                                   |          Owner:  mikeperry
     Type:  project                                                     |         Status:  new      
 Priority:  critical                                                    |      Milestone:           
Component:  Firefox Patch Issues                                        |        Version:           
 Keywords:  tbb-fingerprinting, tbb-rebase-regression, MikePerry201304  |         Parent:           
   Points:                                                              |   Actualpoints:           
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Comment(by mikeperry):

 gfx.downloadable_fonts.fallback_delay governs how long until fallbacks are
 used. Unfortunately no values of that pref fully disable the behavior (see
 nsFontFaceLoader::StartedLoading()).

 It looks like the fallback might be governed by callers of
 gfxUserFontSet::FindFontEntry(). Two possiblitities for further
 investigation are gfxFontGroup::FindPlatformFont() and
 gfxFontGroup::ForEachFontInternal() and their subsequnt calls to
 gfxPlatform::ResolveFontName() and
 gfxPlatformFontList::FindFontForFamily(). Both set
 gfxFontGroup->mSkipDrawing if it's not yet time to use the fallback
 font... The bad news is it looks like you can still compute the width of
 the fallback font even before it is actually drawn (see
 gfxTextRun::Draw()).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/8455#comment:6>
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