Things have improved a little bit, but I still can't use InDesign with justified Khmer text. Here are the current errors (at least what I see so far). InDesign 16.4 2021
1) Some words are still breaking on lines even though there is not a zero-width space (words should only break if there is a zero-width space).
2) Some special characters still break, but they should ALWAYS stay with the previous character and never break on a line without the previous word.

Please use the Khmer Unicode block to make sure no symbols are missed: https://unicode-table.com/en/blocks/khmer/ Thanks!

So it turns out, if you select the option "Fully Justify" for "Single Word Justification" all the issues, except for the special characters being broken apart from the lines they should be on work. So it is very close to being perfect! Just need those special characters to stay on the same line as the words they are part of.

Does InDesign use ICU for the text layout engine?

@Nathan I tried to generate that odd line breaking issue and it seems to happen when those punctuations are followed or preceded by Khmer numerals. It should be something needs to be done from the text engine side.

But a special unexpected bonus - it looks like I can get spelling check to work!

@Abhinav Thanks so much for the update! It appears while justification is much better than before, some of the same issues still occur. First, line-breaks occur on special characters when they should not (parenthesis, colons etc. should not be allowed to break but should stay with the character they are next to). Also, in certain cases, words still go outside of the margins. And it appears hyphenation no longer works (it works in CS 5.5). If you need any files from me, don't hesitate to ask. I've attached some screen shots.

Should I also mention that I have also just discovered that khmer script justification is working normally with hum hum QXP... Which I sadly have stopped using years ago, because of its price. I did not see myself learning it back again, I don't have that much time, and I am so used to inDesign, but if I have another big book project in khmer language, I might have to reconsider!

Yes, the Adobe World-Ready Paragraph is definitely bugged and broken with khmer script. The letters are exploded inside and outside of the text boxes margins when justifying. Strangely enough, this does not happen when you use the regular Adobe Composer, but then the subscripts are broken (except for one font which seems to work, the Khmer OS MetalChrieng). So for now, the only solution is to align to the left and use manual spacing to justify. It is of course very time consuming to do it on each page.
It is also quite sad that this bug has not been addressed for 6 years now! Especially knowing that it was working before. I wish there was a way just to use the former composer engine with the current version of InDesign.

Like Nathan mentioned, it has been an issue since 2013. I’m facing the issue on both CC 2017.1 (12.1.0.56 Build) and CC 2019 (14.0.1 Build).

This is broken from CS 6 all the way to the latest version of InDesign CC 14.0.1 @Abhinav

Please fix Khmer Unicode font in Adobe ID. Thanks.

It is really annoying with this Khmer justification problem.

I already voted it for advocate adobe "fixing Khmer font usage in adobe Indesign" ! I had used this app for almost 8 years for design book, the error of JUSTIFY Paragraph still have problem! and some fonts not yet properly used it.

This has been an issue since 2013 - I filed a bug with Adobe - #3613633 but it still has never been fixed. This is a regression bug, as Khmer justification works in CS 5.5
Would be great to have this fixed so we can use the current edition of InDesign to publish in Khmer.