Custom HiDPI cursor on MacOS

Description

I’ve noticed that winit doesn’t have an option to set a custom HiDPI mouse cursor on MacOS. Most MacBook and iMac screens are scaled x2, even the mouse cursor is doubled. I think it’s worth considering because a custom cursor is currently blurred on retina displays.

I took a look at the implementation and managed to achieve a sharp cursor by dividing the size of the NSImage by 2. By keeping the physical representation, I only changed the logical representation. It’s worth mentioning that the hotspot also needs to be changed. For testing, I set it to 0. This is just a test, not a final solution.

File src/platform_impl/apple/appkit/cursor.rs

    let image = unsafe {
        NSImage::initWithSize(NSImage::alloc(), NSSize::new((width / 2).into(), (height / 2).into()))
    };
    unsafe { image.addRepresentation(&bitmap) };

    let hotspot = NSPoint::new(0 as f64, 0 as f64);

What do you think about it?

Relevant platforms

macOS

Read src/platform_impl/apple/appkit/cursor.rs first, focusing on how the NSImage size and cursor hotspot are currently created. Reproduce the custom-cursor behavior on a Retina macOS display, then verify that the cursor is sharp and that its hotspot remains correct after the logical-size change.