Hi James,
Browser extensions, not WordPress plugins.
Again, do you have the same issue with all externsions temporarily deactivated? (that includes any ad/tracker blockers, which can be a bit overzealous sometimes)
If not, re-activate each extension one by one until you find the extension causing the problem.
Ha! Forgive me for being so obtuse. I only had two and disabling them changed nothing.
I am quite certain that at the end of the day, we will discover that this is a “jQuery is missing” problem. Why? I have recently, and quite accidentally, discovered that there is exactly one tiny change that can expose or dispose this problem: “define( ‘SCRIPT_DEBUG’, true/false );”.
define( 'SCRIPT_DEBUG', false ); Problem
define( 'SCRIPT_DEBUG', true ); No Problem
I suspect that enabling “SCRIPT_DEBUG” is compensating for some dependency that is failing to load jQuery. I only believe this because WordPress tells me that javascript must be enabled when I know damn well that JavaScript is already enabled. I suspect that the test to determine if JavaScript is enabled is a jQuery function that does not return as expected, hence my conclusion that jQuery is not there. I haven’t looked into the code to confirm this, mostly because I have no idea, yet, where to look. WordPress code is pretty gnarly for a novice. There are other clues that lead me to this conclusions, but the site-editor.php complaint is the minimum case.
Thanks for the help,
Chris.