The X Window System and the curse of NumLock

May 14, 2024

In X, like probably any graphical environment, there are a variety of layers to keys and characters that you type. One of the layers is the input events that the X server sends to applications. As covered in the xlib manual, these contain a keycode, representing the nominal physical key, a keysym, representing what is nominally printed on the key, and a bitmap of the modifiers currently in effect, which are things like 'Shift' or 'Ctrl' (cf). The separation between keycodes and keysyms lets you do things like remap your QWERTY keyboard to Dvorak; you tell X to change what keysyms are generated for a bunch of the keycodes. Programs like GNU Emacs read the state of the modifiers to determine what you've typed (from their perspective), so they can distinguish 'Ctrl-Return' from plain 'Return'.

Ordinary modifiers are normally straightforward, in that they are additional keys that are held down as you type the main key. Control, Shift, and Alt all work this way (by default). However, some modifiers are 'sticky', where you tap their key once to turn them on and then tap their key again to turn them off. The obvious example of this is Caps Lock (unless you turn its effects off, remapping its physical key to be, say, another Ctrl key). Another example, one that many X users have historically wound up quietly cursing, is NumLock. Why people wind up cursing NumLock, and why I have a program to control its state, is because of how X programs (such as window managers) often do their key and mouse button bindings.

(There are also things that will let you make non-sticky modifier keys into sticky keys.)

Suppose, for example, that you have a bunch of custom fvwm mouse bindings that are for things like 'middle mouse button plus Alt', 'middle mouse button plus Shift and Alt', 'plain right mouse button on the root', and so on. Fvwm and most other X programs will normally (have to) interpret this completely literally; when you create a binding for 'middle mouse plus Alt', the state of the current modifiers must be exactly 'Alt' and nothing else. If the X server has NumLock on for some reason (such as you hitting the key on the keyboard), the state of the current modifiers will actually be 'NumLock plus Alt', or 'NumLock plus Alt and Shift', or just 'NumLock' (instead of 'no modifiers in effect'). As a result, fvwm will not match any of your bindings and nothing will happen as you're poking away at your keyboard and your mouse.

Of course, this can also happen with CapsLock, which has the same sticky behavior. But CapsLock has extremely obvious effects when you type ordinary characters in terminal windows, editors, email, and so on, so it generally doesn't take very long before people realize they have CapsLock on. NumLock doesn't normally change the main letters or much of anything else; on some keyboard layouts, it may not change anything you can physically type. As a result, having NumLock on can be all but invisible (or completely invisible on keyboards with no NumLock LED). To make it worse, various things have historically liked 'helpfully' turning NumLock on for you, or starting in a mode with NumLock on.

(X programs can alter the current modifier status, so it's possible for NumLock to get turnd on even if there is no NumLock key on your keyboard. The good news is that this also makes it possible to turn it off again. A program can also monitor the state of modifiers, so I believe there are ones that give you virtual LEDs for some combination of CapsLock, ScrollLock, and NumLock.)

So the curse of NumLock in X is that having NumLock on can be cause mysterious key binding failures in various programs, while often being more or less invisible. And for X protocol reasons, I believe it's hard for window managers to tell the X server 'ignore NumLock when considering my bindings' (see, for example, the discussion of IgnoreModifiers in the fvwm3 manual).

Written on 14 May 2024.
« Some ideas on what Linux distributions can do about the new kernel situation
Turning off the X server's CapsLock modifier »

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Last modified: Tue May 14 23:15:38 2024
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