Insert Media 404 for wpspin_light.gif on subfolder installation

[n7studios]

Using WordPress 3.5 RC3, my Google Chrome Console reports a 404 error for ​http://127.0.0.1/wp-admin/images/wpspin_light.gif.

However, my site is installed in a sub directory at ​http://127.0.0.1/dev

To reproduce:

1. Ensure your browser's console is open to view issues / errors
2. Create or edit a Page
3. Click on the Add Media button
4. Click on the Media Library 'tab' (to the right of Upload Files, below the Insert Media title)
5. Click on an image
6. Console says:

GET ​http://127.0.0.1/wp-admin/images/wpspin_light.gif 404 (Not Found) - load-scripts.php:2

Visiting ​http://127.0.0.1/dev/wp-admin/images/wpspin_light.gif displays the image as expected.

.media-sidebar .settings-save-status .spinner {
	background: url(../../wp-admin/images/wpspin_light.gif) no-repeat;

Yeah, we can't do that.

Rawr. Let's copy wpspin_light.gif over to wp-includes/images. And maybe just name it wpspin there, because dark doesn't exist anymore, anyway.

In 23023:

Copy wpspin_light to wp-includes as wpspin.gif. Use it in media views. see #22728.

Can someone verify this on a retina device?

In 23026:

Move HiDPI block to the bottom of media-views. @media does not provide additional specificity, so the non-retina stuff overrides if the retina rules come first. props lessbloat, fixes #22728.

I don't get it. Why is it necessary to copy the image? "wp-includes" and "wp-admin" are in the same level, even in subfolder installs, aren't they? Am I missing something?

We should never be guessing at paths - this bites us every so often. Folders can be moved if so desired, etc.