Alright, I have enabled per-page discussion just for this.
The way the collapsible sidebar works on mobile is that you open an anchor to current_url#side-bar which then displays the sidebar; in order to close the sidebar, you jump to current_url## which hides the sidebar again.
Now, the link area is supposed to be the entire screen minus the sidebar. For this purpose, a (semi-)transparent image acting as link is set to cover the whole viewport, but with a negative z-index, so it is displayed behind the sidebar. This image could technically be anything, but for simplicity, a single black pixel is used. Considering it is only 119 Bytes, anything else would probably have been bigger to load in. It's not perfect, but this is WikiDot, Ladies and Gentlemen.
Sadly, I can't answer why we have been hotlinking the SCP wiki's pixel for years. But I do know that Cad broke the closing functionality in the Revision 162, when he added the separated language box, where he accidentially put the closing anchor inside the sidebar's div, making it being rendered only in there (which reduced the clickable area to a single pixel, making it virtually impossible to interact with for touch screen users).
TL;DR: Pixel important. Mobile Sidebar was broken, now isn't. WikiDot bad.
Eschew elucidation, espouse obfuscation.
