The Arabic letter "ha" does not appear correctly with some web fonts. Testcase https://dl.dropboxusercontent.com/u/95157096/85f61cf7/ge5de3bwr3.html Screenshot https://dl.dropboxusercontent.com/u/95157096/85f61cf7/sqhx3rwx3d.png Steps to reproduce: 1. Close Firefox, if it is open. 2. Go to "Control Panel", open "Region and Language", change the "Format" to Arabic, then press OK, see https://dl.dropboxusercontent.com/u/95157096/85f61cf7/1iu1zssfl3.png. 3. Open Firefox, then go to https://dl.dropboxusercontent.com/u/95157096/85f61cf7/ge5de3bwr3.html. The problem occurs with harfbuzz enabled. See https://dl.dropboxusercontent.com/u/95157096/85f61cf7/bvk3htubts.png.
I can reproduce this problem since the Nightly build 2011-08-15, Regressed by bug 667166.
This is a problem with the implementation of the 'locl' (Localized Forms) feature in the font. It didn't show up under Uniscribe shaping, because Uniscribe doesn't support localized forms. With harfbuzz enabled, however, we support localized forms, and this font has an arab:ARA lookup that changes the HA glyph before normal Arabic shaping has a chance to take effect. You can reproduce the problem without setting the Region & Language format to Arabic if you include the lang="ar" attribute in the testcase, so that the Arabic-language forms are used instead of the "default" OpenType language system. Explicitly disabling the localized forms feature, with -moz-font-feature-settings: "locl" off; in the CSS, would work around this issue.  See http://www.microsoft.com/typography/OpenTypeDev/arabic/intro.htm#tags: "Currently, the Uniscribe engine only supports the "default" language for each script."
This also happens with the fonts https://dl.dropboxusercontent.com/u/95157096/85f61cf7/ectqnqpde4.html and https://dl.dropboxusercontent.com/u/95157096/85f61cf7/nan3v0zmzm.html.
Hardly surprising; I expect Linotype used the same basic pattern for their features across a whole range of Arabic fonts. The same response still applies: what you're seeing is the effect of the localized-forms feature for the Arabic language (OpenType tag 'ARA'), which uniscribe didn't apply, but harfbuzz does.
This bug has been fixed in bug 1108616.
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