How to set a single modifier key as a shortcut in openbox?

Question

I want to have my super key start dmenu. I have set it as a keyboard shortcut in my rc.xml as follows:

<keybind key="0xffeb">
      <action name="Execute">
        <command>dmenu_run</command>
      </action>
</keybind>

I tried specifying it in the key attribute as W, W-, and 0xffeb, but none of these worked.
W responds to pressing the letter w, and the others appear to do nothing.

I want the shortcut to trigger when the super key is pressed and released on it's own. Is this possible?

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This is cross posted from super user as per the guidelines here. I've read this question: Super key as shortcut - Openbox, but I didn't see any useful information in it.

Answer 1

I ended up using xcape, a utility designed to do exactly this:

xcape allows you to use a modifier key as another key when pressed and released on its own. Note that it is slightly slower than pressing the original key, because the pressed event does not occur until the key is released.

^{Quoted from the xcape readme}

Using xcape, you can assign the press and release of a modifier key to a different key or even a sequence of keys. For example, you can assign Super to a placeholder shortcut like ⎈ Ctrl⇧ Shift⎇ Alt SuperD with:

xcape -e 'Super_L=Control_L|Shift_L|Alt_L|Super_L|D'

Now when you press and release Super without pressing any other keys, xcape will send keyboard events simulating presses of ⎈ Ctrl⇧ Shift⎇ Alt SuperD (holding all the modifier keys down as if you pressed them like a shortcut).

If you press Super and another key (or hold Super too long, the default timeout is 500 ms), xcape will pass the keyboard events through as is, without firing extra keys.

If you put the placeholder shortcut in rc.xml, it will run when Super and only Super is pressed.

<keybind key="C-A-S-W-d">
      <action name="Execute">
          <command>dmenu_run</command>
      </action>
</keybind>

Other shortcuts involving Super will not be affected.

Note that you'll have to run xcape each time you boot, so you may want to put it somewhere like ~/.config/openbox/autostart where it will be run automatically.

Answer 2

Run xev and press your super key to get the appropriate key name. Mine is Super_L according to the output:

KeyRelease event, serial 48, synthetic NO, window 0x3600001,
    root 0x91, subw 0x0, time 1648471775, (167,-8), root:(927,138),
    state 0x50, keycode 133 (keysym 0xffeb, Super_L), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

Assuming yours is the same, this should work with a single key press of the super key (aka windows key):

<keybind key="Super_L">
      <action name="Execute">
        <command>dmenu_run</command>
      </action>
</keybind>

Save the file and run killall -SIGUSR2 openbox to reload the configuration.

As an aside, I am working in the config file for the local user at $HOME/.config/openbox/lxde-rc.xml. Using W only registers as the super key when used in combination, but Super_L works on its own instead of being read as the letter w.