Post metaboxes jump when you hit the bottom of the page

[melchoyce]

See this video: ​https://cloudup.com/cBpjxUro7_x

(Also attaching the video, just in case)

Tested on OSX Chrome.

[stephdau]

Stating the obvious: that because of the bottom-pinning brought in b4 vs Chrome's elastic scrolling.
I think we can solve that with somewhat of a max-bottom based on the document's bottom edge.
The value will need to be calculated live, since the page's total height can change via user interactions (auto-expanding editor, meta box drag-n-drop between columns, tag adds, etc).

Looking into a patch.

[stephdau]

So the problem is:

• the bottom of the sidebar is pinned, with a max-bottom of 180px: https://core.trac.wordpress.org/browser/trunk/src/wp-admin/js/editor-expand.js?rev=29628#L470
• but when one scrolls passed the doc's bottom edge with elastic scrolling, we re-enter the " let it scroll" mode, with an "auto max-bottom": https://core.trac.wordpress.org/browser/trunk/src/wp-admin/js/editor-expand.js?rev=29628#L444

[azaozz]

29349.patch fixes it here. Also adds a var for the 'bottom' distance and fixes pinning of the sidebar when the height is nearly the same as the viewport height.

[nacin]

Go for it.

[azaozz]

In 29656:

Editor expand:

• Adjust calculation when comparing sidebar position to wpfooter top. Fixes an annoying jumping when the user scrolls off the bottom.
• Use a var for the 'bottom' distance of the sidebar.
• Fix sidebar pinning when it's height is nearly the same as the viewport height.

Fixes #29349