Gallery layout block adds all media items when changing an image

[MikeLittle]

## Steps to reproduce:

1. Have multiple items already added to the media library.
2. Create a new post
3. Drag a the Gallery pattern (two images side by side)
4. Select one image
5. Click media library
6. Select ONE image
7. Click Add to gallery
8. All images are selected
9. Click update gallery
10. All images are added to the gallery

## Expected behaviour
Only one image is added to the gallery

I'm not sure whether the bug is in the block or the media library. There were no errors or warnings in the JS Console

Running WordPress under 'wplocal' (flywheel/wpengine).
PHP 7.4.1
Browser: Chrome 88.0.4324.192

Video demo of the issue: ​https://vimeo.com/522746498/0ca3c5a137

[desrosj]

Good catch on this one!

I did some testing, and looks like this is reproducible in trunk as well. However, it only happens if the original two images are in the gallery. For example, if you replace one of the images and then click Media Library as seen in the video, the issue does not occur.

My hunch is that it has something to do with both IDs being null in the block definition (<!-- wp:gallery {"ids":[null,null],"linkTo":"none","align":"wide"} --> is what gets inserted).

Some other things I noticed:

• If you replace one of the default images and then add to the gallery, it only shows two images in the media modal when there are actually three.
• After clicking on an image in the gallery and clicking the edit pencil in the top right, the buttons and links within the highlighted area cannot be selected.

This will probably need an issue over on GitHub, but leaving this open for now for visibility.

[desrosj]

I've created ​https://github.com/WordPress/gutenberg/issues/29835 and ​https://github.com/WordPress/gutenberg/issues/29834 upstream for these issues. Going to leave this open for now for tracking purposes.

[peterwilsoncc]

This was fixed in the package updated committed in [50678] for ticket #52912.