scrolling does not work once mouse is grabbed

The title pretty much told it all. It occurs in Windows 10 RS1 on qemu 2.7.0. Interestingly, I can scroll in the guest if the mouse is not grabbed. So using usb-tablet sort of works around it, but if I explicitly grab the mouse with Ctrl+Alt+G, scrolling will also stop working.

The host is Arch Linux so the qemu build uses gtk(3) for GUI by default. I wanted to test with sdl but it seems sdl support in qemu is sort of broken that I can't even start the virtual machine properly with that.