Steps to Reproduce

1. Visit https://commons.m.wikimedia.org/wiki/Special:NewFiles?mobileaction=beta (in beta mode)
2. Scroll down and start scrolling up until the blue back to top button shows

1. Tap an image
2. Inspect the page and notice where the back to top button is in the background (the CSS class is backtotop)
   
3. Tap the details button where it overlaps with the back to top button

Expected Results

• Image details are shown

Actual Results

• Nothing happens

Environments Observed

• Mobile Commons

Browser Version

• Chromium v70.0.3538.67

OS Version

• Ubuntu v18.10

Device Model

• Desktop

Device Language

• English

Developer notes

The z-index of backtotop and overlay are identical, but .backtotop is a child of body, whereas the overlay is a child of #mw-mf-viewport.

.overlay class z-index should be bumped higher, so it is higher than backtotop.
There needs to be 4 categories of z-index would be "article", "above article", "overlay", "above overlay" not 3.

@z-indexBase: 0;
@z-indexOverBase: 1
@z-indexOverlay: 2;
@z-indexOverOverlay: 3;

We might want to consider changing the HTML structure as part of this change so that backtotop is more closely associated with the HTML of the article to which it belongs.