Installed LanguageTool for Libre Office Writer, but find LT even more useful when filling out web site message fields, like this one, in Firefox. In Writer, the squiggly red underline is enhanced, and grammatical errors are flagged by LT with a different color squiggly underline. In most text entry windows in Firefox, the underlines are solid, and clicking on the word or phrase brings up alternative spellings or edits, or occasionally a suggestion of fixing things.

With squiggly underlines, right-click on the word as usual to get a drop-down menu of corrections. With solid underlines, left-click on it to get spelling, grammar, or style corrections or suggestions. I recommended LT for anyone who wants more than built-in spell checking.