through the help of the delete block, i have made a discovery. if you have a branch on the bottom left that grows off-screen, if you use the delete on a floating selection block that appears right above the bottom right side of the screen, you can somehow delete the end of the branch on the left. this also goes for placing left-facing branches on it.
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.