Enter a character with a typo in LO Online Calc's formula bar, eg. "tset", and hit Enter (note how it already shows the suggestion, "test" below the word). => The suggestion replaces the word, and remains in the formula bar. Tested with Safari 10.1.2.
This seems to be much better now, however it only works in formula bar, and not in a cell. Additionally, if you type a typoed word, eg. "tset" in the formula bar, and correct it to test, it gets in IME mode, but any further typing resumes normal typing, so getting in IME mode is unnecessary.