@ John, I had the same problem and found that there were issues with Bulletproof Plugin when Update to 3.5 was installed.
The Advice I found was deactivate Bulletproof-Plugin, deinstall it and reinstall it – I did so and no change.
But my mistake was – deinstalling the Bulletproof Plugin probably does NOT remove the .htaccess file from the Admin Folder – this step has to be done before deinstalling from the Bulletproof – Panel.
Today I did just this –
1.removed the wp-admin .htaccess file within Bulletproof
2.opened a post saved it and the loooong editor was gone.
3. reinstalled the .htaccess file with Bulletproof.
So if ever you use the Bulletproof Plugin you can do so – or just check if you have a .htaccess file in the admin folder (maybe from another Plugin ( not verified ) just rename it in your File manager to i.e. old. htaccess then open a post / page in your Dashboard and save it.
If this works you can rename the .htaccess back and try editing sth. again – hope it helps to solve some problems