input: Add "AltGr" alias for ISO level3 shift modifier

[Trundle]

It's what's written on keyboards, so perhaps a bit more accessible to users.

[YaLTeR]

cc @sodiboo who had the original concern.

So, I'm not sure this is such a good idea, because you need a specific xkb option (lv3:ralt_switch I think), or I guess a specific layout, to make AltGr actually emit the level 3 modifier. By default (on us with no special options) it emits a right alt, which does not trigger the level 3 modifier. (Compared to Win/Super, which sends that key by default.) So I think it would only be confusing to make AltGr an alias here.

Unless of course it is established practice in compositors to accept the AltGr alias for this.

[Trundle]

The key is alt right on us layouts, but the key is also labeled Alt and not AltGr on US keyboards, isn't it? My non-scientific quick image search found keyboards with Alt and AltGr though 🙈

As another data point, the de layout maps AltGr to ISO Level3 Shift.

Re other compositors: In my limited experience, it's Mod5-ish everywhere

[kchibisov]

Mine AltGr was always a regular Alt on US layout, yes it's called AltGr on keyboard, but it's just alt. And when you compose it's a Compose key, not AltGr, so whether it's ISO level3 shift is a layout dependant, I guess.

Keep in mind that there's also a level 5 shift.

[sodiboo]

because you need a specific xkb option (lv3:ralt_switch I think), or I guess a specific layout, to make AltGr actually emit the level 3 modifier

I don't think the "layout" needs to be as specific as you think -- i've used the norwegian layout my whole life, that's what is physically printed on my keyboards whenever possible (really "nordic" usually) and the only option i ever set for xkb is layout "no"; for me, on both my computers, this will lead to the AltGr key emitting the ISO_Level3_Shift keysym and my other alt key emitting the Alt_L keysym -- no special options here!

As far as i can tell by looking at a picture of a US layout, emitting Alt_R is correct there because those keys don't say AltGr, they say just Alt. so given that the configured keyboard layout matches the physical layout, i don't think there are any layouts where (with no special options) AltGr emits Alt_R or right alt emits ISO_Level3_Shift