urgent:
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* make mouse drag intertia more accurate and responsive
* fix boundaries to prevent windows from getting stuck


needs more ephysics support:
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* window stacking


features:
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* horizontal gravity (EASY)
* configurable restitution/friction (bounciness+elasticity of collision) (EASY)
* gravity based on accelerometer?
* collision effects
* window client collision notification for app collision effects
* mass based on window client cpu load
* explosion effect for new windows to knock nearby windows out of the way
* window rotation using physics (HARD)
???