"properties" in WindowData (https://cs.chromium.org/chromium/src/services/ui/public/interfaces/window_tree_constants.mojom?type=cs&q=window_tree_consta&sq=package:chromium&l=25) seems to be used to set rect and size (https://cs.chromium.org/chromium/src/ui/views/mus/native_widget_mus.cc?sq=package:chromium&dr=C&rcl=1481905173&l=1077 and https://cs.chromium.org/chromium/src/ui/views/mus/native_widget_mus.cc?sq=package:chromium&dr=C&rcl=1481905173&l=910). Right now all use cases are through NativeWidgetMus. Since NativeWidgetMus is going away soon, wait until then to figure out how these properties will be used and if they need to be scaled for high-dpi mode.