How can I change an underutilized keyboard key into a modifier key

Question

I have a Korean keyboard that looks like this:

Hanja (한자), just to the left of space, is so little utilized even when typing in Korean, yet it's in prime real real estate for my left thumb as an i3 window manager mod key. That will free up Alt for use in my code editor.

My question is: How can I turn the Hanja key into a modifier key?

I verified the key with xev:

KeyPress event, serial 32, synthetic NO, window 0x2200001,
    root 0x281, subw 0x0, time 11955968, (-132,626), root:(491,646),
    state 0x0, keycode 131 (keysym 0xff34, Hangul_Hanja), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 32, synthetic NO, window 0x2200001,
    root 0x281, subw 0x0, time 11955968, (-132,626), root:(491,646),
    state 0x20, keycode 131 (keysym 0xff34, Hangul_Hanja), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

Keycode 131 = 0x83

From the output of xmodmap, I see that mod3 is available.

shift       Shift_L (0x32),  Shift_R (0x3e)
lock        Caps_Lock (0x42)
control     Control_L (0x25),  Control_R (0x69)
mod1        Alt_L (0x40),  Alt_R (0x6c),  Meta_L (0xcd)
mod2        Num_Lock (0x4d)
mod3      
mod4        Super_L (0x85),  Super_R (0x86),  Super_L (0xce),  Hyper_L (0xcf)
mod5        ISO_Level3_Shift (0x5c),  Mode_switch (0xcb)

I tried xmodmap -e "add mod3 = Hangul_Hanja" resulting in the following...

shift       Shift_L (0x32),  Shift_R (0x3e)
lock        Caps_Lock (0x42)
control     Control_L (0x25),  Control_R (0x69)
mod1        Alt_L (0x40),  Alt_R (0x6c),  Meta_L (0xcd)
mod2        Num_Lock (0x4d)
mod3        Hangul_Hanja (0x83)
mod4        Super_L (0x85),  Super_R (0x86),  Super_L (0xce),  Hyper_L (0xcf)
mod5        ISO_Level3_Shift (0x5c),  Mode_switch (0xcb)

... after adding set $mod Mod3 to my i3 config file and restarting, I ended up with no modifier key whatsoever, i.e. it didn't work.

Also, the Hanja key isn't behaving like the typical modifier key. If I press and hold the key it creates repeated keystrokes, whereas the typical modifier key does not.

I hope there's some keyboard experts in the house that can offer some of their expertise.

Linux Mint 18 (Cinnamon) i3 Window Manager

Answer 1

X11 modifier handling is a bit peculiar. You can't take any key and declare it to be a modifier: some things have to be consistent between the modifier settings and the keysym settings.

The classic modifiers are Shift, Control, Alt, Meta, Hyper and Super (with two keysyms each, e.g. Shift_L and Shift_R) (plus Caps Lock and Num Lock which behave somewhat differently). Most applications won't work with other keysyms used as modifiers.

So to use the Hanja key as a modifier, make it be Hyper_L (for example).

keycode 131 = Hyper_L
remove Mod4 = Hyper_L
add Mod3 = Hyper_L

This makes the key no longer used as the Hanja key at all. There may be a way to keep both functions if you use XKB instead of xmodmap, but I don't know how.