The dimensions of the rectangle into which the application will be drawn, in physical pixels.
When this changes, onMetricsChanged is called.
At startup, the size of the application window may not be known before Dart code runs. If this value is observed early in the application lifecycle, it may report Size.zero.
This value does not take into account any on-screen keyboards or other system UI. The padding and viewInsets properties provide a view into how much of each side of the application may be obscured by system UI.
- WidgetsBindingObserver, for a mechanism at the widgets layer to observe when this value changes.
Size get physicalSize => _physicalSize;