Innovations in user interface (“UI”) features of an electronic gaming device, and in features of backend processing to implement the UI features, are presented. For example, an electronic gaming machine (“EGM”) can selectively transfer target symbols between multiple windows. Each of the windows uses a set of reels, which spin in the respective windows. When at least a threshold count of target symbols lands for a reel in a window, a stack of target symbols is transferred to a corresponding reel in each of one or more other windows. After target symbols are transferred, a bonus feature, special mode, or other supplemental feature can be triggered depending on the counts of target symbols in the respective windows. As another example, an EGM can selectively start a supplemental feature (such as a bonus feature or special mode) in multiple windows depending on a count of target symbols in the windows, collectively.