[HarfBuzz] Windows 10 monbaiti.ttf upgrade (5.01 -> 5.51) caused loss of diacritical marks when shaped with harfbuz

[Behdad Esfahbod]

Ok, after discussion with MS people, I pushed a fix out.  Please test.

On 15-12-04 07:51 PM, Jonathan Kew wrote:
> On 4/12/15 17:14, Tom Hindle wrote:
>> Hi,
>>
>> I noticed in windows 10 that when using System Mongolian Baiti Font,
>> text shaping via harfbuz (seen in firefox + chrome) no longer displayed
>> the dots bellow the words.
>>
>> Tested using hg-view.exe using harfbuz 1.1.2
>>
>> Attached pngs of output from hb-view using both the windows 10 font
>> (5.51) and the windows 7 font (5.01).
>>
>>
>> hb-view.exe Windows7Font_monbaiti.ttf --text-file=Mongolian.txt
>> outtput-format=png --output-file=file-monbaiti_5.01-harfbuzz-1.1.2.png
>>
>> hb-view.exe Windows10Font_monbaiti.ttf --text-file=Mongolian.txt
>> outtput-format=png --output-file=file-monbaiti_5.51-harfbuzz-1.1.2.png
>>
>> Installing the Windows 7 font on windows 10 fixed the behavior in
>> harfbuz based application (eg. firefox).
>>
>> Windows 10's IE (11) + Edge both displayed the dots regardless of which
>> font was used.
> 
> 
> This appears to be a result of changes to the 'calt' feature in the font.
> 
> With the older font, applying   --features calt=0   when shaping will give you
> the dotless forms; i.e. the dots are being provided by the
> (applied-by-default) Contextual Alternates feature.
> 
> But with the newer font, the behavior is reversed; applying  --features
> calt=0  will give you the dotted forms, while   calt=1   (the default) results
> in dotless.
> 
> So you should be able to control the result (in Firefox or Chrome) by setting
> font features appropriately in the CSS; but the fact that the older and newer
> fonts behave differently makes this problematic if you want content to render
> the same across Windows versions.
> 
> (I also tried to tweak the font features in Edge, but AFAICT it ignores my
> attempts to apply (-ms-)font-feature-settings to Mongolian content.)
> 
> Whether this is a deliberate change, a font bug, or harfbuzz mis-applying the
> lookups in some way is not yet clear to me.
> 
> JK
> 
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