For U+1880..U+1886 Uniscribe thinks they are non-joining.
For U+1887 Uniscribe thinks it's joining, but looks wrong to me.
For now, match Uniscribe.
/* Mongolian joining data is not in ArabicJoining.txt yet. */
if (unlikely (hb_in_range<hb_codepoint_t> (u, 0x1800, 0x18AF)))
{
+ if (unlikely (hb_in_range<hb_codepoint_t> (u, 0x1880, 0x1886)))
+ return JOINING_TYPE_U;
+
/* All letters, SIBE SYLLABLE BOUNDARY MARKER, and NIRUGU are D */
if ((FLAG(gen_cat) & (FLAG (HB_UNICODE_GENERAL_CATEGORY_OTHER_LETTER) |
FLAG (HB_UNICODE_GENERAL_CATEGORY_MODIFIER_LETTER)))