Last week or so, when I get into an SE site (MO in particular), a popup window appears in the lower left corner for few milliseconds. Some kind of clickable button there, with not enough time for me to figure out what it is. On different SE sites it is different but looks similar in design.
Do others encounter this too? Does anybody know what is it and how to get rid of it?
Strictly speaking this belongs to the whole SE meta, please migrate it if it is off-topic here.