Improve the "disaster recovery" mode

[azaozz]

We've had a "disaster recovery" that saves post title, content and excerpt in the browser's sessionStorage for a while. It runs every 15 seconds on the Edit Post screen and just before the user submits the form. However it shows very rarely, only when it detects that saving failed, and if the "There is a newer autosave..." message is not present (so we don't show two recovery options at the same time and confuse the users).

This behavior needs to be changed as the content in the browser is more recent. If there is a "newer autosave" message and the disaster recovery script activates, it should hide the message. There are some other improvements like not running editor.save() if the content in the editor has not changed, etc.

A great improvement will be to be able to show some sort of a preview with the changes that will be restored. However that deserves its own ticket and is probably out of scope this time.

In 37025.patch:

• Hide the "More recent autosave available" notice if present.
• Simplify the "Backup available" notice (no inline undo), and add short explanation of what is changing and how to revert it. (The text may need some editing.)
• When restoring to the Visual editor make sure it creates an undo level.
• When restoring to the Text editor use document.execCommand() so the change can be undone with Command/Ctrl + Z. (Using textarea.value = 'string' prevents the user from undoing it).

Other suggestions about how to make this a bit more user friendly welcome :)

How to test this (on a test install):

• Start a new post.
• Type something.
• Save a draft.
• Type something more.
• Wait for about 10 seconds for the in-browser autosave to kick in. It runs every 15 seconds.
• Reload the page.

Couple of seconds after the page loads the "The backup of this post in your browser is different from the version below." notice should show. (The couple of seconds delay is needed to make sure the post content is fully loaded in TinyMCE so we can compare it to the content in sessionStorage. This reduces false positives a lot).

In 37025.1.patch:

• A bit longer help/explanation of how to use this.
• Added some subtle animation when removing an existing autosave message and opening this message.

In 37737:

Autosave: improve the notice when the sessionStorage autosave is different than the content.

• Make it higher priority than the server autosave.
• Change it so the editors undo and redo can be used.
• Replace the restore link with a button.
• Add better explanation/help.

See #37025.

I've ran into this a couple of times now while testing other things: I refresh the page, try to put my cursor in the editor and it starts sliding down. That's pretty annoying.

@azaozz What's missing here? Can we close it if it's only testing?

Replying to ocean90:

Seems to be working well. New tickets for any bugs please.