Meanwhile, a year later.

Updated the test html to 'cover' all skins (mostly increasing dimensions and offset).

!!! If testing on production wiki, please *DO NOT SAVE* but use preview as we don't want to hand the below hack on a plate for someone to find and abuse.

Content:

<div style="
 position: absolute;
 top: -180px; left: -10%;
 z-index: 99999;
 width: 120%; height: 400px;
 background: rgba(0, 0, 255, 0.8);
 outline: 1px solid orange;
 font-size: 2em;
 text-align: center;
">
<div style="
 position: absolute;
 top: 0; right: 0; bottom: 0; left: 0;
 overflow: hidden;
 background: rgba(255, 255, 255, 0.8);
 outline: 1px solid blue;
">[[File:Transparent.gif|4000x4000px|link=//example.org|Click]]</div>
<div style="position: absolute; bottom: 0; text-align: center;">Navigation/Search</div>
</div>

<div style="
 position: absolute;
 top: -170px; left: -190px;
 z-index: 99999;
 width: 240px; height: 640px;
 background: rgba(0, 255, 255, 0.8);
 outline: 1px solid red;
 font-size: 2em;
 text-align: center;
">
<div style="position: absolute; bottom: 0; text-align: center;">Logo/Sidebar</div>
<div style="
 position: absolute;
 top: 0; right: 0; bottom: 0; left: 0;
 overflow: hidden;
 background: rgba(255, 255, 255, 0.8);
 outline: 1px solid green;
">
[[File:Transparent.gif|350x350px|link=//example.org|Click]]
</div>
</div>


Current mediawiki/core.git@master:

== Vector skin ==
- Logo:       Not safe
- Sidebar:    Not safe
- Personal:   Not safe
- Tabs:       Not safe
- Search:     Not safe
- Sitenotice: Not safe
- Page name:  Not safe

== Monobook skin ==
- Logo:       Safe
- Sidebar:    Not safe
- Personal:   Safe
- Tabs:       Not safe
- Search:     Safe
- Sitenotice: Not safe
- Page name:  Not safe

== Modern skin ==
- Logo:       Not safe
- Sidebar:    Not safe
- Personal:   Not safe
- Tabs:       Not safe
- Search:     Not safe
- Sitenotice: Not safe
- Page name:  Not safe

Monobook has weird z-index overrides all over the place. For example the
following have an overriden z-index that made them safe for this test:

- li a.selected (the selected tab in the tabs)
- #p-search (portlet for search in the sidebar)
- #p-logo (portlet for the logo in the sidebar)
- #p-lang (porlet for interwiki links in the sidebar)

But the other tabs or the tab bar itself, and the other portlets or the sidebar
itself were all not safe.


After my patch:


== Vector skin ==
- Logo:       Safe
- Sidebar:    Safe
- Personal:   Safe
- Tabs:       Safe
- Search:     Safe
- Sitenotice: Safe
- Page name:  Safe

Tested to confirm proper compatibility with jQuery UI and VisualEditor dialogs and overlays etc. (e.g. personal portlet stays behind the overlay that mutes the page, as it should).