Chrome Version: 71.0.3556.0
OS: 11042.0.0

Vietnamese Telex currently lacks the standard transform rule whereby typing one of the tone mark triggers (s, f, r, x, j - for acute, grave, hook-above, tilde, dot-below tone marks respectively) should assign that tone mark to applicable preceding letter, effectively modifying existing tone mark if any. 

Which letter exactly to apply the new tone mark depends on whether strict or flexible diacritic placement mode is active. This should only affect tone mark diacritics, leaving letter modifier diacritics intact.

Non-exhaustive examples:
- type "casf" -> expect "cà", currently get "cáf"
- type "cangsf" -> expect "càng", currently get "cángf"
- type "caosf" -> expect "cào", currently get "cáò"

NOTE: When flexible diacritic placement is active, this should work correctly for *all* cases as per Vietnamese orthography, with regard to diacritic placement and syllable structure, not just the minimal simplistic examples above. The Telex rule itself, if implemented correctly as such, is linguistically sound and thus consistent and reliable.