I've no technical background in typography, but as a use case when the user type 'A[ZWNJ]B' the editor should show 'A[Discontinuation of continuous script but without any space or kerning]B'. It can be translated to '[the end shape of A][No space or kerning][the beginning shape of B]' . Persian/Hebrew/Arabic scripts have different glyphs for the same character based on their position in the word (beginning, middle, end), so the ZWNJ is vital here. Regarding ZWNJ from user point of view in these scripts, it works exactly like 'Space' but without showing it.
This might be misunderstanding on my part, but It is strange to me if the font (or shaper?) replaces the ZWNJ with space. It's OK to show nothing for ZWNJ but not the space. I've not such experience with other editors such as Gedit (or even with terminal emulators) and if this is the case how other editors figure it out?
Thanks a lot,
P.s. Regarding the new patch I'll test it as soon as possible.