FSE: Modified Template Part block attributes trigger a blank revision in the page editor

[apeatling]

I am seeing this notice on my home page in the editor almost 100% of the time:

When I look at the revision it wants to restore I'm seeing this blank revision:

We should investigate this and make sure it's not a bug. It's possible I am not saving the home page enough, since I've mainly been editing the header template part content. However, seeing a blank revision gives me pause, that shouldn't happen.

[noahtallen]

So I'm really digging into this to see if I can ever get a blank revision. Unfortunately, not yet! I think I know why we get the message almost 100% of the time, though.

Two conditions have to both be true to get into the loop:

1. You click "leave" after it tells you you have unsaved changes.
2. When you click leave, it had recently done an autosave of those unsaved changes.

To reproduce:

1. Open a page, and just click into the placeholder where it says "start writing or type / to choose a block":
   That inserts a new paragraph block even if you don't type anything
2. Wait for one minute. At the one minute mark, it should trigger an autosave.
3. Click the back button OR "edit header" (either one will take you away from the page)
4. It will prompt you that you have unsaved changes to see if you want to leave anyways. Click "leave" without saving or publishing or anything like that.
5. Immediately go back to the page and you'll see the autosave revision message. With just clicking into the placeholder, I get the smallest revision I can reproduce so far:

Every time you go back to the page, it will tell you about the autosave revision until you manually "update" or "publish" or "save" the post.

I know that I click leave without saving all the time while I'm debugging these pages, and since we navigate around so much while debugging without updating or publishing, we get the message all the time.

While that isn't truly a bug (it's working as designed), I think it just highlights how poor the autosave flow is. My intuition would be that it always restores the latest autosave when going back to the page in the spirit of always having your latest changes, just like all our modern, realtime apps (i.e. google docs) always have your latest changes. That's pretty far out of the scope of this issue, though!

[noahtallen]

I wonder if the blank revision could be an edge case where gutenberg (or whatever mechanism) falsely thinks the post is dirty, so the autosave happens, but there aren't any changes to compare in the diff. No clue what that edge case would be though. I'm also trying to modify things like block attributes to see if that would trigger something like that.

[noahtallen]

Yay, I was able to trigger a blank revision! You can do it by:

1. Open a page with FSE enabled
2. Select the header template part block. In the sidebar, add an anchor and hit enter:

3. Open the sidebar again to make sure the anchor is still there
4. Wait at least one minute for the autosave Note, make sure you didn't make any changes other than the anchor
5. Navigate away from the page (and click "leave" when it warns you of unsaved changes)
6. Go back into the page and see the revision notice.
7. Click the link to see the revision. It will be blank.

Restoring the autosave does not restore your unfortunate anchor tag :(

[noahtallen]

It should no longer be possible to reproduce this, but feel free to reopen if you notice it again :)