BUG: Fix new page form clearing when selecting a Under another page option

[bergice]

Fixes #2339

Revert d05e142

EDIT:

In the end this didn't revert but instead resolved the issue by creating an empty container for the React loading component to mount inside rather than mounting in a container that already had a potentially significant DOM tree.

[robbieaverill]

Does this mean we lose all the loading animations again? I’d rather have them than fix this bug, if I had to choose

[bergice]

Does this mean we lose all the loading animations again? I’d rather have them than fix this bug, if I had to choose

Yep.

On closer look I can see this is caused by 2f2fb2b due to ReactDOM.render clearing the content of the container the component is rendered in:

ReactDOM.render() controls the contents of the container node you pass in. Any existing DOM elements inside are replaced when first called. Later calls use React’s DOM diffing algorithm for efficient updates.
https://reactjs.org/docs/react-dom.html#render

Will update PR.

[robbieaverill]

Boom

[ScopeyNZ]

Ok I literally prepared this exact patch for another issue I've encountered where gridfield-orderable-rows is not working.

[ScopeyNZ]

The test failures are pre-existing in 1.3: https://travis-ci.org/silverstripe/silverstripe-admin/builds/462576472

They're being looked at separately.