I have been with WordPress for 15 years. I am using v.6.5.4. I normally use Classic Editor plug-in but this behaviour happens when Classic Editor is deactivated. I get exactly the same behaviour with the Block Editor. My Theme is very old – Twenty Ten v4.1, but I have nearly 1,000 posts using that old theme.
When I log in to the website, and select posts or pages, and then click on Edit, I get a full screen edit view for about 1 second. I then get a reduced width view, as if for a tablet or mobile phone. All the functionality is there and works, but it is much harder to work on the content.
The theme view is set to Desktop. The screen options in the edit screen are all ticked except Slug, Two Column is ticked, and distraction free is also ticked.
I suspect there is a change to the Tiny MCE editing environment but I have not been able to find it. I have the TablePress plug-in working which also uses Tiny MCE, but that is still working fine.
Dick Nathan
Good view (for 1 second)
The page I need help with: [log in to see the link]