HUION PenTablet devices are graphics tablet devices aimed at artists. These tablets tend to aim for the lower end of the market, driver support is often somewhere between meh and disappointing. The DIGImend project used to take care of them, but with that out of the picture, the bugs bubble up to userspace more often.
The most common bug at the moment is a lack of proximity events. On pen devices like graphics tablets, we expect a BTN_TOOL_PEN event whenever the pen goes in or out of the detectable range of the tablet ('proximity'). On most devices, proximity does not imply touching the surface (that's BTN_TOUCH or a pressure-based threshold), on anything that's not built into a screen proximity without touching the surface is required to position the cursor correctly. libinput relies on proximity events to provide the correct tool state, which again is relied upon by compositors and clients.
The broken HUION devices only send BTN_TOOL_PEN once whenever the pen first goes into proximity and then never again until the device is disconnected. To make things more fun, HUION re-uses USB ids, so we cannot even reliably detect the broken devices and do the usual approach to hardware-quirking. So far, libinput support for HUION devices has thus been spotty. The good news is that libinput git master (and thus libinput 1.9) will have a fix for this. The one thing we can rely on is that tablets keep sending events at the device's scanout frequency. So in libinput we now add a timeout to the tablets and assume proximity-out has happened. libinput fakes a proximity out event and waits for the next event from the tablet - at which point we'll fake a proximity in before processing the events. This is enabled on all HUION devices now (re-using USB IDs, remember?) but not on any other device.
One down, many more broken devices more to go. Yay.