Composer content editor is unusable

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v8.5.4
Updated from v8.5.2 a week ago, and am just now adding a new article using Composer. The Content editor seems to have a 7 second timeout when "Source" is selected. As soon as one clicks into the content, you have 7 seconds to do what you want to do before being flipped out of "Source" mode.

 
fischershaw replied on at Permalink Reply
I take it no one else is seeing this?

Might this have something to do with autosave? If so, how do I disable it, or at least set it for a long, long periodicity?

Second possibility is that I neglected to update the composer.json file, and there is some wrong version of something running. Is it safe to apply the composer.json & lock files after having done the update?

Edit:
I turned off autosave in config/concrete.php and that fixes the problem.