Comment by JonathonW

ICU (International Components for Unicode) provides an API for this: http://userguide.icu-project.org/boundaryanalysis

Assuming Blink is using the same technique for text selection as V8 is for the public Intl.v8BreakIterator method, that's how Chrome's handling this-- Intl.v8BreakIterator is a pretty thin wrapper around the ICU BreakIterator implementation: https://chromium.googlesource.com/v8/v8/+/refs/heads/master/...