[regression] Table Drag handles no longer respond to up/down arrow keys

Problem/Motivation

The table drag handles are no longer keyboard-accessible. These are used in many places in the content-authoring and site-building UIs.

When table drag handles have focus, the up/down arrow keys should re-order the table rows. This has stopped working. It was working as of D8.0.0-beta10, and works in D7.43.

Where the table rows have a tree structure (e.g. menu UI) the left/right keys let a user change the indented (parent) relationship. The left/right keys are still working as expected.

Steps to Reproduce

Note: if testing with macOS, you'll need to make sure the tab key is configured to cycle focus through all links and controls - follow these Detailed instructions.

1. Install 7.43
2. Admin > Structure > Menus > Management >list links
   /#overlay=admin/structure/menu/manage/management
3. Tab a bunch until focus is on the arrow(handle) for SubMenuItem (i.e. Dashboard)
   

Up Arrow

1. Focus on Dashboard
2. Press up arrow
   
3. left/right arrow doesn't do anything

Down Arrow

1. Focus on Dashboard
2. Press down arrow
   
3. left/right arrow doesn't do anything

Left/Right Arrow

1. Focus on Dashboard
2. Press down arrow a lot to get it under Taxonomy.
3. left arrow does something
   
4. right arrow does something
   

Proposed resolution

Restore the old keyboard behaviour:

• When a drag handle has focus, up-down arrow keys change the vertical order of the table rows.
• When drag handles are used on tree structures, the left/right keys can be used to control indentation.

Remaining tasks

• Find out where the regression happened. - DONE. It was broken by commit dcf9ab4, between 8.0.0-beta11 and 8.0.0-beta12.
• Fix it - restore old keyboard behaviour.

User interface changes

None as such. This is about restoring keyboard-accessible behaviour that stopped working. No changes are intended for pointing devices such as mice.

API changes

None.

Data model changes

None.