also i tried going 1 to the right because that was an option for some reason- and this happened- 
edit 1: and suddenly i can place a left-facing branch on top of the already existing left-facing branch??
edit 2: tested it, the mushroom stayed, and the off-screen branch disappeared and i can no longer select to delete the mushroom. looks like it's stuck there!
edit 3: so. i deleted the branch under it. it floats now. have not gotten another mushroom yet, will try to place one over the floating one next chance i get.
edit 4: it worked, the mushroom has regained collision with the help of another mushroom.