tree.min.css is requested twice on every page view for a logged-in user

Reproduction steps

1. Log in as a user who has edit right on the page
2. Visit http://localhost:8080/xwiki/bin/view/Sandbox/
3. Open the browser debugger and make sure that "disable cache" is unchecked
4. Click on the "Sandbox Test Page 1" link and observe the network tab

Expected

tree.min.css?evaluate=true is requested at most once per page view.

Actual

It is requested twice, and since it is served with Expires: Thu, 01 Jan 1970 00:00:00 GMT neither response can be reused. That is 2 × 30.6 KB = 62 KB transferred on every single page view.

For comparison, the total amount transferred on a warm-cache page view is about 87 KB, so this one stylesheet accounts for roughly 70% of it.

A guest requests it only once, which is the clue to the cause.

Analysis

The two requests have different initiators:

• the <link> emitted in the page itself, for the tree used by the Navigation panel and by the export tree modal;
• a second one injected from JavaScript by CKEditor/InlineEditor, which is loaded in view mode by the in-place editing preload (see the companion issue about that preload).

Hence the difference between a guest and a user with edit right.

Note that the fact this stylesheet is not cacheable at all is a separate, already reported problem: XWIKI-22935. The two issues compound each other — the duplication only costs bandwidth on every page view because the response cannot be cached — but they can be fixed independently.

Suggested fix

Avoid the second injection when the stylesheet is already present in the page. Fixing the in-place editing preload so that CKEditor is not loaded in view mode at all also removes this second request as a side effect.