I did't fully read the documentation and misunderstood this API. Scene.windowIdealSize(_:) is for zooming in macOS. (The command "Window" > "Zoom" can trigger it.) This also explains why there exists WindowIdealSize.maximum, I think. For my needs, I used Scene.defaultSize and people would see my designed window size on first-time window presentation. WindowGroup { Color.accentColor.opacity(1/4) .frame(minWidth: 500, minHeight: 500) .frame(idealWidth: 800, idealHeight: 800) } // For window zooming (a macOS feature), // which can be accessed via double-clicking the window title bar (see Settings). .windowIdealSize(.fitToContent) // For window size on first presentation .defaultSize(width: 640, height: 640) In this example, people will see 640640 window for the first time. Also, when people zoom this window, it expands to 800800, not as large as it can.

Problems gone 16.4 beta (16F1t). Seems working fine on all macOS.

Problem persists and behaves in the same way in a fresh macOS 15.5 beta (24F5042g). I used UTM as a Virtualization environment. bringing approximately 2 watts of power load to my Mac I'm new to Apple silicon therefore I wrongly measured the power consumption. It seems to be 0.15W ~ 0.2W on M4 and 6W on virtualized M4.