So even now, after more than 15 years of using X11, I can apparently run into user-facing corner cases that are weird and look like bugs. I just got an x230 ThinkPad to serve as a portable laptop where my trusty T61 is too bulky.

It has a particularly moronic keyboard. In particular, the menu key has been replaced by PrintScreen, which is appreciated by exactly nobody:

As before, I want to map this to Super_R to talk to my window manager. So I do just that, naively:

xmodmap -e "keycode 107 = Super_R"

This appears to work in notion, but in some ways it doesn't. It feels like this key is not recognized as the mod4 modifier at all times. After some debugging, this was proved correct. The modifier status before and after the xmodmap invocation above looks like this:

$ xmodmap -pm | grep mod4

mod4        Super_L (0x85),  Super_R (0x86),  Super_R (0x87)

The values in () are keycodes. Note that the keycode I just added (107 == 0x6b) is missing. Apparently when adding new modifier keys, you need to re-add the modifier after setting up the new keycode. So what I was supposed to do above was this:

xmodmap -e "remove mod4 = Super_R" -e "keycode 107 = Super_R" -e "add mod4 = Super_R"

With that I do see the new keycode in the mod4 map, and things work correctly:

$ xmodmap -pm | grep mod4

mod4        Super_R (0x6b),  Super_L (0x85),  Super_R (0x86),  Super_R (0x87)

I don't doubt this is a well known behavior, but I really wish it was less user-hostile and I didn't have to spend over an hour tracking this down.