The post content shifts to the right when publishing

[afercia]

Describe the bug
When publishing a new post, the whole post content shifts to the right producing an unexpected movement of the whole content section.

As a user, I'd be very confused by things moving while I publish and I'd wonder if something went wrong.

Having the whole editor unexpectedly moving doesn't seem an optimal user experience.

GIF:

To reproduce
This happens only on WordPress trunk, with or without the Gutenberg plugin activated. Doesn't happen on 5.3.2.

• create a post
• make sure the Gutenberg Settings sidebar is expanded
• enter a title
• enter some content
• click Publish
• the content area shifts to the right
• the publish sidebar appears
• click the Publish button within the publish sidebar
• the content area is still shifted to the right
• close the publish sidebar by clicking on the X button
• the content area shifts back to its original position

Expected behavior
To not see the whole editor area move

Additional context

This seems related to the changes from #18044

Previously, the Editor content area had a right margin equal to the sidebar width. With the refactoring from #18044 the margin is gone.

[draganescu]

I can confirm this behavior. It is not easy to add the margin back, as now the sidebar is removed from dom and the post content has no special class to select for that situation. And I think the problem is that the sidebar should not be removed by the publish panel being open, it should simply remain under it.