New check: Arabic letter high hamza isn't classified as a mark.

Many fonts incorrectly treat ARABIC LETTER HIGH HAMZA (U+0675) as a variant of
ARABIC HAMZA ABOVE (U+0654) and make it a combining mark of the same size.

But U+0675 is a base letter and should be a variant of ARABIC LETTER HAMZA
(U+0621) but raised slightly above baseline.

Not doing so effectively makes the font useless for Jawi and
possibly Kazakh as well.

Added to the Universal Profile
com.google.fonts/check/arabic_high_hamza (experimental)
(issue fonttools#4290)

CHANGELOG.md

@@ -10,6 +10,7 @@ A more detailed list of changes is available in the corresponding milestones for
 ### New checks
 #### Added to the Universal Profile
   - **EXPERIMENTAL - [com.google.fonts/check/arabic_spacing_symbols]:** Check that Arabic spacing symbols U+FBB2–FBC1 aren't classified as marks. (issue #4295)
+  - **EXPERIMENTAL - [com.google.fonts/check/arabic_high_hamza]:** Check that glyph for U+0675 ARABIC LETTER HIGH HAMZA is not a mark. (issue #4290)
 
 ### Changes to existing checks
 #### On the Universal Profile

Lib/fontbakery/profiles/universal.py

@@ -819,6 +819,51 @@ def com_google_fonts_check_spacing_symbols(ttFont):
         yield PASS, "Looks good!"
 
 
+@check(
+    id="com.google.fonts/check/arabic_high_hamza",
+    proposal=[
+        "https://github.com/googlefonts/fontbakery/issues/4290",
+    ],
+    rationale="""
+        Many fonts incorrectly treat ARABIC LETTER HIGH HAMZA (U+0675) as a variant of
+        ARABIC HAMZA ABOVE (U+0654) and make it a combining mark of the same size.
+
+        But U+0675 is a base letter and should be a variant of ARABIC LETTER HAMZA
+        (U+0621) but raised slightly above baseline.
+
+        Not doing so effectively makes the font useless for Jawi and
+        possibly Kazakh as well.
+    """,
+    experimental="Since 2023/Oct/20",
+    severity=4,
+)
+def com_google_fonts_check_high_hamza(ttFont):
+    """Check that glyph for U+0675 ARABIC LETTER HIGH HAMZA is not a mark."""
+    import babelfont
+
+    ARABIC_LETTER_HIGH_HAMZA = 0x0675
+
+    passed = True
+    font = babelfont.load(ttFont.reader.file.name)
+
+    if "GDEF" in ttFont:
+        class_def = ttFont["GDEF"].table.GlyphClassDef.classDefs
+        for glyph in font.glyphs:
+            if set(glyph.codepoints).contains(ARABIC_LETTER_HIGH_HAMZA):
+                if glyph.name in class_def and class_def[glyph.name] == 3:
+                    passed = False
+                    yield FAIL, Message(
+                        "mark-in-gdef",
+                        f'"{glyph.name}" is defined in GDEF as a mark (class 3).',
+                    )
+    # TODO: Should we also validate the bounding box of the glyph and compare
+    #       it to U+0621 expecting them to have roughly the same size?
+    #       (within a certain tolerance margin)
+
+    if passed:
+        yield PASS, "Looks good!"
+
+
 @check(
     id="com.google.fonts/check/required_tables",
     conditions=["ttFont"],