When finding a grammatical error which I consider to be irrelevant, permissible, or a false positive, I have the option of right-clicking it and hitting "Ignore"
The problem is that, in my case, the blue line DOES disappear, but only for a minute or so. After having typed a bit and encountering the same grammatical error, Languagetools will again underline the original error, even though I had told it to ignore it. Thus, I would have to wait till the end of my writing before deciding what to ignore.
Thanks for your report. The problem might be that IgnoreGrammarErrorAt() in sw/source/core/edit/edlingu.cxx is called and removes the error, but LT will re-send it when it get called again. LibreOffice then probably just adds the error again. It's not even easy to ignore this instance of the error, as the text might change. Basically LO would need some text property that says which errors are to be ignored for a text range. The other solution might be to change "Ignore" to "Ignore Rule" and then call LT's ignoreRule() method.
LightProof is affected, too.
See https://bugs.freedesktop.org/show_bug.cgi?id=56954
This has been fixed at LibreOffice (https://bugs.freedesktop.org/show_bug.cgi?id=56954), so I guess I can close this issue.