a question about the MS renderer.
I’m debugging an Arabic font, and in the sequence رّكك the Shadda on the final Reh “moves” with the kerning between final Reh and initial Kaf. By "moving" I mean as I type Reh + Shadda the mark is placed correctly, but as I continue to type Kaf the kerning between Reh and Kaf is applied, but the Shadda gets "pushed to the right" by as much as Reh and Kaf are (negatively) kerned.
Here a couple of fonts in Microsoft Word for Mac 16.16.5; the bottom one being the font I'm debugging.
I tested further, and to my astonishment it seems that any mark “moves” like this, affected by kerning when its base glyph is kerned against something to follow. In the font I am testing, final Reh and initial Kaf have their overlap kerned, so the Kaf arm slides nicely over the Reh. When I looked how some other fonts avoid the marks moving with kerning I found that e.g. in this example initial Kaf simply had a negative right sidebearing; makes sense to do it like this in the first place.
What does not make sense to me is why the renderer would behave like this. Applying kerning, but pushing the marks above kerned pairs to the side from their original place above the base glyph. Is my font buggy, or is this really the renderer “working as intended”?
Anybody have insights to this or encountered a similar issue with Word or other Microsoft programs?