Hello. Recently, Adobe
announced its official support for 5 Asian scripts including Khmer in Photoshop (and Illustrator). I'm not sure if that includes InDesign, but it renders Khmer script well through World-Ready Paragraph Composer until the paragraph justification is set. I tried to change the leading, the spacing, and any other option available, but I couldn't get it working.
See the image below.
- Adobe InDesign CC 2017 (also tried in CC 2018, the same results.)
- Adobe World-Ready Paragraph Composer
- Zero-width spaces are used in the text to mark the word boundaries, somehow important for Khmer text.

I also opened a bug report here:
https://indesign.uservoice.com/forums/601180-adobe-indesign-bugs/suggestions/36534535-khmer-script-justification-is-broken, but no reply.
I hope this could reach people from Adobe. Please correct me if I said anything wrong.
Comments
Of course at the lower level of font support there is CoolType, which provides access to fonts and OpenType features and the like. Occasionally something may in fact be a limitation of CoolType itself. But mostly people have issues “above” that.