Font Forum

I tried to modify a glyph of built-in font; arabtype.ttf. I reduced advance width by shrinking the glyph and keep LSB, RSB constant. It's seen س letter of medial form. I tested it with 5 different preceeding letter, so they showed incorrect contextual join; all joins are gapped. How to solve this kind of problem? I do really need your help, many tahnkfulness in advance...

You might need to update the anchor(s), but if that doesn't work you might have to update corresponding OpenType features.

Of course, if you make the glyph contracted horizontally, without changing RSB, LSB naturally you will get gaps beside this glyph from both sides right and left. Move the RSB 'guide' and the LSB 'guide' and make them touch the glyph in order to eliminate the gaps.
Wherever you have joining arabic glyph (cobtextual) you SHOULD make the RSB and/or LSB thouch the joining side of the glyph.
All the best.

There is a feature designed for cursive fonts where the letters are supposed to join together in a continuous stream. I forget the exact name but basically they are anchors but not for the positioning of diacritic marks.

If you use this feature (I must admit I have never had occasion to use this feature yet) then each glyph has an 'entry' point and an 'exit' point and when it is rendered then the horizontal position is adjusted so that the exit point of one glyph corresponds to the entry point of the next glyph.

If I were in your position I would be trying to find out about 'Cursive Anchors' or 'Cursive Positioning' in open type features.