Thai line breaking: how to break Thai text effectively
Situation with Thai text on a client site is that we can't control where exactly particular words/sentences are going to break between the lines (how web browser will handle it). Often, content appearance is indicated as incorrect by local reviewers.
Workaround
to this is that copywriter needs to deliver Thai content with breaking ​ and non-breaking  zero-width-space chars included.
In practice, rather than:
ของเพื่อนๆ ที่ออนไลน์อยู่
we should use something as ugly as:
ของเพื่อนๆ​ที่​ออนไลน์อยู่
The above is just an example, I don't really know where exactly the breakpoints are allowed.
In fact, non-breaking zero spaces alone would do the trick either ... it's just more strict and correct to use breaking ones as well for better accuracy.
And while it definitely is doable like this, it also is a time consuming and not very effective solution for a large site content management. Simply said, the effort put into it doesn't match the effect needed.
Research so far has lead to the workaround mentioned, looking for a better way how to handle this. Even W3C doesn't have a solution yet and is just discussing whether it should be part of CSS3 specification.
Thai language utilizes spaces very rarely, mostly to distinguish between sentences etc. Therefore, common appearance of a Thai sentence is one looong string. Where to break such a string when more lines of text are put together is determined by particular words identification. For words identification local dictionaries are used which are most probably part of operating system or web browser, I'm not entirely sure about these.
Apparently, the more web browsers / operating systems you check on the more results you get! Moreover, there's not much you can do about this as it's system driven and there are no "where to break Thai" settings available.
Using <wbr/>, ​ or ­ to indicate where the breakpoints really are won't prevent web browser thinking (even though wrong) that some breaks are also possible in places, where you haven't defined them e.g. in the middle of a word which might be grammatically incorrect.
If such a word is placed at the end of a line (depends on screen resolution, copy length, CSS rules defined) and the browser applies his wrong line breaking rule on it then you would end up with a Thai line breaking issue, no matter that you have defined another breakpoints before, after or somewhere else in the word - browser will always use a breakpoint that he thinks is closest to EOL, not just the ones you have gently suggested by inserting one of the mentioned chars in your markup.
That's why you actually need to focus on where not to break your text (non-breaking zero-width-space), not where it's allowed. And that's what lead us back to the ugly and long markup example in the "Workaround" section above. That way a line break can strictly only occur where you have allowed it to be, but it's messy.
Any other solution how to handle this more effectively would be appreciated ... and who knows, it might even help W3C in their implementation?
THANK YOU!
Answer
I know this thread was quite some time but I have something to say as a native Thai. I read lots of Thai web pages everyday and I feel the quality of Thai line breaking by the modern web browsers nowadays is perfectly acceptable.
As I know, Google Chrome browser uses ICU4C, Internet Explorer uses Uniscribe API, and Firefox uses libthai to break Thai sentences into words. For Thai people I know, how these web browsers handle line breaks in Thai is perfectly acceptable for them. (actually we used to have this problem with very early version of Firefox (1.x) but that is resolved now.)
Thai line breaking and word breaking, unlike western languages, is still considered an unsolved problem and is still actively tackled by many linguistics researchers. Currently there is no implementation that could perfectly break a sentence to Thai words. IBM ICU Boundary Analysis page contains some analysis on this problem.
Many times, it has something to do with the context. For example, the phrase "ตากลม" can be correctly broken to "ตา","กลม" or "ตาก","ลม". Each way says totally different thing but Thai readers can still perfectly understand the intended meaning, given the context.
Given that your local reviewers are already familiar with reading Thai websites, I think maybe they are too pushy on you to resolve this problem. This is common unsolvable problem for all Thai websites, web browsers, and even Microsoft Word.
It is best to wait (or contribute to IBM ICU) until Thai sentence breaking implementation gets better. Let the web browsers handle this. I don't think trying to workaround this problem worth your valuable time. As as I know, even Thai website publishers here just don't care to get this one right.
Should you need to publish a document with a perfect line/word breaking, you may consider other medium, such as PDF document in which you should have more control over the line breaks.
Hope this helps :)