I am using Texmaker 4.5 on Windows 10 and the built-in spell-checker stops working if I copy and paste a section into the document. It seems to work/not-work on a paragraph by paragraph basis. If I try to spell-check a non-working section I am told that no misspelled words were detected, even if I have complete gibberish in the paragraph.
The figure below shows the issue I am having, see how the three gibberish words above the citation are not marked as misspelled, while the words below are correctly tagged with a red line.
All I did was close the bracket and the spell checker stopped working.
I have checked this the same document on my laptop running texmaker 4.1 on a Linux platform and it works perfectly.