PointerBreak is a macOS app that puts a skateboard behind your cursor. It trails cartoon exhaust smoke, launches off ramps, grinds rails, carves halfpipes, and stays wherever it lands until you go and collect it.
The constraint that shapes everything is that the cursor isn't mine to move. You move it. So the board can't just be a physics body — it has to be decoupled from the thing it's chasing.
It ends up with four states. Grounded, it exponentially chases the cursor and eases its pitch towards the direction of travel, folded so the wheels never end up on top. Riding, grinding or piping, a surface holds it and the cursor carries on doing whatever you're doing. Airborne, a ballistic integrator has it, and more furniture can catch it on the way down. Resting, it sits where its airtime ran out until your pointer passes back over it.
My favourite part is how the tricks are solved. Airtime is fixed at the moment of launch, so the trick is chosen there and the spin rates are worked out backwards to complete exactly on landing. Much easier than simulating forwards and hoping it comes out level. Flips are whole turns and shove-its whole or half turns, so every trick finishes flat, and landing takes the facing from the direction of travel — nothing left to correct on touchdown.
It asks for no permissions at all, which surprises people. There's no injection into other apps and no plugin, because it doesn't need to be one: being everywhere is a property of the window. Accessory activation policy so it's never the active app, borderless and click-through so your clicks reach whatever is underneath, screen-saver window level so it draws above the menu bar, and one window per screen so it spans multiple displays properly.
Even the idle behaviour is shaped by that. After two seconds of stillness it drops the display link and polls the mouse location at 10Hz instead — deliberately a poll rather than a global event monitor, because a global monitor is exactly the thing that triggers a permission prompt.
It's Swift 6 and Metal. Everything is simulated on the CPU and drawn as instanced unit quads from one shared buffer — smoke, skid marks, furniture and the board all in one pass. The effects are signed distance fields in the fragment shader; the board and furniture sample bundled sprite atlases and fall back to their original distance field shapes if an atlas is missing.
It's free, with a one-off unlock for the layout editor and scoring.