Joy, this was really helpful thank you. I have tested with Firefox on Ubuntu and the problem does not exist, I can customise my WordPress site.
I only have the problem with Safari on Mojave.
SCRIPT_DEBUG did not fix the issue.
I have identified the following Java errors
parsing the ‘sandbox’ attribute:’allow-orientation-lock’, ‘allow-presentation’ are invalid sandbox flags. attr query.js4:11269
TypeError: Argument 1 (‘target’) to MutationObserver.observe must be an instance of Node. observe wp-emoji.js:164
TypeError: null is not an object (evaluating ‘document.body.style’). onload site name:568
I could not workout how to export the error to paste here or how to paste a screenshot here so I retyped the above.
Hopefully, someone can point me in the right direction for a resolution.
Thank you for your help.
Regards