I noticed this issue now at least twice on different days. It happens when I have the highlighter on the Review bar double clicked, then start working on the opened PDF file and later change from Okular to let's they the Chrome browser to do some other stuff. When I then return to the opened PDF file to continue reading and highlighting, the highlighter icon on the Review bar is still enabled but you cannot select any text anymore to apply the highlighter.
Is this only with Chrome or happens if you switch to some other app? Can you reproduce it all the time and will all pdf files?
(In reply to Albert Astals Cid from comment #1) > Is this only with Chrome or happens if you switch to some other app? > > Can you reproduce it all the time and will all pdf files? Thanks Albert for following up! Since I have reported this particular issue in September it seems to have disappeared. Despite my very still very intensive use of Okular to annotate PDF files, this issue is not bothering me anymore.