ModifiersChanged message does not indicate keyboard device_id

[maaku]

The modifiers field in winit::event::KeyboardInput is marked as depreciated, and a note in the documentation says that applications should instead handle the ModifiersChanged message (and presumably track the resulting value themselves). However the KeyboardInput event has a device_id field while the ModifiersChanged message does not. Strictly speaking, this is therefore a decrease in functionality.

To illustrate, consider a system that has two keyboards attached--which is not so crazy as it sounds when you consider bluetooth keyboards, or special purpose devices. If a modifier key is pressed on one device, conceivably that could be isolated from the other in the prior model. However with the ModifiersChanged message pressing any modifier key on any device affects the key input of all other devices.

Although this is certainly and edge case, it appears to me to be a bug. Perhaps the ModifiersChanged event type should be augmented to include a device_id?

[Osspial]

device_id in WindowEvent has only ever been implemented on Linux, and from what I can tell it's pretty much impossible to expose on all the other OSes. All the non-Linux backends just pass in a dummy value for that field. As it stands, if you need to distinguish between devices when handling input you should listen for DeviceEvents instead of WindowEvent.

In light of that we've been moving towards removing DeviceId from WindowEvent completely (see #1374), but that PR needs more design work before it can move forward.