I am using Writer 4.2.1.1 to type Lao document on Mac OSX, when I use Opentype fonts e.g Saysettha OT or Phetsarath OT the lines do not break correctly (it breaks at space not word like I use Saysettha MX)
Lao Script is not needed anymore. LibreOffice ships with ICU (International Components for Unicode) Lao support, so line breaking and word tokenization happen automatically - no need for ZWSPs except when you need more fine-tuned control (such as working with minority languages that use Lao script). Lao spell check is also available, though it needs improvement. If you find words missing, be sure to report them to the ICU team on their bug report/feature request system. They can add missing words fairly easily. The words should also be added to LibreOffice dictionaries (dictionaries/lo_LA at master · LibreOffice/dictionaries · GitHub). If you are having trouble, make sure your settings are correct (CTL should be set to Lao, and you may have to install Lao Spell Checker separately depending on the platform you are using).
It appears that the Lao Script Windows extension (or use of Zero Width Space, U+200B) is required to obtain correct breaking for Lao text. This is what it states under the Add-on Functions / Break heading:
Insert breaks between Lao words and syllables in the selected text, with optional hyphenation of dictionary-listed words, to allow correct wrapping of Lao text. For Unicode fonts, the break inserted is the “Zero-Width Space” character (U+200B, ZWSP), recognized by most applications as a discretionary line-breaking character. For Writer, inserted ZWSP characters can be displayed or hidden on screen by pressing Ctrl F8, and they never appear on printed documents. Although in Calc the ZWSP characters are always visible on screen, they do not appear in printed documents.
It is very likely that future upgrades to LibreOffice/OpenOffice will include display time wrapping of Lao text, making insertion of break characters unnecessary. However the Check function (see below) will still be useful for checking Lao spelling.