Many people use 2 spaces after each full stop before the next sentence. This is often considered better formatting and appearance. The grammar checker has no way to be told to accept double spaces after the full stop, so whole documents are littered with blue underlining.
There should be an *option* in the grammar checker to say whether sentences will have 1 or 2 spaces after a full stop (and before a new sentence beginning with a capital letter). It is good to have help maintaining consistency in a document. Having the grammar checker enforce a choice is helpful, but it is essential to have an option for 2 spaces.
Thank you for your report and the solid reasoning!
Double spaces after full stops are considered as completely outdated in many languages (e.g., in German and French), but they are still used sometimes in English and maybe other languages; cf. TeX (and LaTeX) typesetting. Therefore such an option (!) would be useful.
Setting the Status of this bug report to NEW, changing the Importance to “enhancement” (as appropriate) and changing the Summary to make it clearer.
Perhaps you'd like to use the LanguageTool (www.languagetool.org) extension for grammar checking instead. It only complains when you have 3 spaces after a full stop.