Why is none needed here? That's surprising.

The checkboxes are always present in the DOM, but are only shown when .blocklyMenuItemSelected is applied below. Since they're absolutely positioned here, they're laid out atop the menu items like so:

That means that when the mouse moves over one, the menu attempts to resolve which item it's a child of, but their position doesn't correspond to their corresponding menu items, so the one that's being hovered over might actually be a child of an earlier menu item than where the mouse is, hence the jank. I just deleted this entire rule and moved height/width into the selected case below to make it clearer; now they're still always present in the DOM, but have no bounds (and hence won't be mouse move targets, or be absolutely positioned in such a way that they aren't children of the menu item they're displayed above) which still solves the problem and is hopefully clearer.