List view drag and drop - make it possible to drop at all levels of the hierarchy

[talldan]

Description

At the moment, there's a known issue when dragging and dropping in List View. It's only possible to drag a block where there's an adjacent block at the position you wish to drop to.

For example, in the following structure, a paragraph can't be moved to after the group because the drag and drop system uses the presence of other blocks to determine the drop target:

Technically this is difficult to achieve because List View is currently implemented as a flat table.

Step-by-step reproduction instructions

1. Create a nested block structure with no blocks after the parent block
2. Try dragging and dropping one of the nested blocks after the parent

Expected behaviour

It should be possible to drop a block after the parent

Actual behaviour

This isn't possible if the parent block is the last block in a list, only when there's another a block after the parent block

Video

CleanShot.2022-02-24.at.13.14.55.mp4

[jasmussen]

Google Keep has a somewhat unique handling of nested items, which could potentially inspire some behaviors in the list view. Specifically it embraces the fact that it's a list, as opposed to a complex layout as is seen in the canvas. Nesting an item simply requires you to use the drag handle and pull it rightwards, literally indent it with the gesture:

Similarly, moving a nested item leftwards, literally un-indenting it with a gesture, un-nests it.

This assumption that dragging an item rightwards is an intent to nest an item under the preceeding item likely can't translate 1:1 to a list of actual blocks with content, but sharing in case it inspires adjacent ideas.

[zaguiini]

It's not only un-nesting... It's also not possible to move a block to the end of a group. It always becomes a child of the last node, instead of becoming the last node itself. It sounds confusing, so let me add a recording:

Screen.Recording.2021-12-15.at.16.02.05.mov

Look how I can make Group the previous sibling of Layout Grid, but I cannot make Layout Grid the next sibling of Group. It always ends up inside Group.

[talldan]

Thanks @zaguiini, yes that's another instance of the issue. It's hard to describe!

[skorasaurus]

Been wondering about this for a while, I thought I was doing something wrong because I couldn't figure out how to do that; relatively frustrating and still present in Gutenberg 13.8.1

[andrewserong]

I've opened a potential fix over in #49742 to explore this.