If the Composer browser window is not high enough to show the whole properties panel, the bottommost button will be cut off.
1) Resize Composer browser window to be not very high.
2) As expected, tree panel covers some of properties panel in right side menu
3) Scroll properties panel all the way down
Properties panel scrolls all the way, all buttons are visible.
Properties panel won't scroll all the way, bottommost few tens of pixels remain hidden behind tree panel.