Typing the word "سري" in the Rich Text demo using the KacstOne font, the inter-letter spacing between the last two glyphs (the leftmost) should shrink as the font has negative kerning there, but what happens is that the space expands as if the kern were applied positively.

However, saving the file and opening it again will show the text in its unkerned status (no kern is applied at all) which still incorrect.

The problem can also be seen when comparing the string "رس رس رس رس" as rendered with KacstOne in the Rich Text demo against other technologies or applications.

This problem does not occur when using the same font with Pango or Microsoft Word, or when using other fonts like Traditional Arabic, Times New Roman or Tahoma.