Allow resizing the individual content areas in Appearance / Menus

[namith.jawahar]

The space to select posts, pages and custom posts for dropping into the menu is restricted to 200px height which makes it really difficult to select posts/pages when creating/editing menus.
The user experience will be greatly improved if the section can be resized.

[namith.jawahar]

Resizable Menu

https://core.trac.wordpress.org/ticket/60763

[NekoJonez]

Two things, I would like to see what #Design and #Accessability think here so nothing goes wrong here.

Also, it that feature only for the first pages div or all of them?

[jorbin]

Milestoning so that this can be considered for 6.6. I would be interested in hearing if there are any design concerns with implementing this.

One note is that the resize property is only supported by 82% of browsers but I don't think that needs to be a blocker ​https://caniuse.com/css-resize

[joedolson]

It's worth noting that the resize property is supported by 98% of desktop browsers, and this issue isn't addressing the mobile UI for editing menus, which may not be helped significantly by this change, given the limited screen space generally.

On the accessibility front, while the resize handle doesn't have any keyboard accessibility built-in, that shouldn't be a blocker; it will be exactly the same experience as it is right now. This will improve things for some users, and should not make things worse for any users.

[namith.jawahar]

Replying to NekoJonez:

Also, it that feature only for the first pages div or all of them?

It targets all the divs, works for pages, custom post types, tags and everything else that might come in there. For things like Custom Links we can use a css selector to not show the resize option if preferred, but not implemented in this PR.

[joedolson]

I tested this and it works well and doesn't appear to interfere with the design in any noticeable way. I think this should move forward.

I'm removing the accessibility focus, however, as this has already received accessibility feedback and does not have any accessibility concerns.

[Joen]

Looks like the resize property is restricted to being vertical, so as a desktop affordance for this particular interface, it doesn't seem harmful to add.

[joedolson]

This looks good. I checked over the other containers impacted by the CSS; the taxonomy tab panel divs in the classic editor, and they do not appear to be effected by these changes.

I'll add a comment in the CSS to explain the 75px calc value, but otherwise this is good to go.

[joedolson]

In 58256:

Menus: Add resize: vertical to menu item selector panel.

Make the panels for selecting menu items to add to a panel resizable using the CSS resize property. This minor change makes it easier to select items to add on desktop devices, and has nominal impact on other interfaces.

Props namith.jawahar, NekoJonez, jorbin, joen, joedolson.
Fixes #60763.

In r58256