What does the blobby icon in Hexcells Infinite do?

On the initial screen of Hexcells Infinite, there are 3 icons in the lower left-hand corner:

The first one on the left controls music & sound effects, the middle one lets you swap what the mouse buttons do, but what does the third one do? It's normally white and almost invisible, but it becomes the gray blob pictured when you move the mouse over it. Clicking it toggles an outline around it, but surely it does more than that.