The in-between inserter shows up when you hover between two blocks, and disappears when you move the cursor out from that area:

However a number of actions can cause this inserter to linger and stay, presumably because mouse action to fade the inserter out again isn't fired. One easy way to reproduce this is is to use the keyboard shortcut to open the list view, control alt O, while the inserter is present:

Other ways to make the inserter linger is when keyboard shortcuts are used to resize the window, or to invoke the web inspector. It also happens if the viewport resizing gets invoked:

Quite simply, if you move the mouse out from canvas itself fast enough, it will stay around. Occasionally this can cause horizontal or even double scrollbars to appears, as the canvas size is reduced while a stray sibling inserter lingers in the canvas (#30055).

To address this and fade out the sibling inserter in these cases, can we add a timeout that checks if the mouse is over the sibling inserter, and fade it out if not? Are there other solutions?