Keyboard Concepts

wezterm allows assigning action(s) to specific key events, and comes pre-configured with a number of commonly useful assignments.

This page describes how key presses are handled and turned into actions or sent to the terminal as text.

It's important to understand these concepts when considering keyboard input; first, some operating system concepts:

  • Input Method Editor (IME) - An OS-provided service which allows for rich composition of input, often with a pop-over candidate selection window. This is commonly used for Asian input, but on some systems the IME may also be responsible for emoji input or dead keys. The IME may have multiple modes per language and those modes can be changed dynamically.
  • Keyboard Layout - An OS configuration that describes how to translate physical key button presses into inputs appropriate to the user's preferred input locale. The mapping performed by the layout is largely opaque to applications and, on most systems, can be changed dynamically.
  • Dead Key - a keyboard layout may define these modal keys which don't immediately produce output (and thus appears to be "dead"), but instead holds some state that will compose with a subsequently pressed key. Most commonly used for example in European layouts to produce accented versions of the plain latin alphabet.
  • Physical Key - a way to identify a key based on its hardware-dependent location. wezterm can refer to keys based on code they would emit if configured to use an ANSI US English keyboard layout (even if that layout is not currently active), or based on its raw scan code.
  • Mapped key - a way to identify a key after the keyboard layout has been applied by the OS.
  • Modifier - A key such as SHIFT, CTRL, CMD, ALT that can be held simultaneously while other keys are pressed. Modifier keys are special because keyboard hardware traditionally only supports those four modifiers, and that detail is ingrained into most OS input APIs.

And then some wezterm concepts:

  • Key Assignment - an action assigned to a matching key and modifier combination.
  • Key Table - a grouping of key assignments. For each window, wezterm maintains a stack of table activations, allowing for rich modal keyboard input customization

Keyboard Processing Flow

This schematic depicts the processing flow for keyboard events in wezterm:

flowchart TD
A[OS Generates a Key Event]
A --> B{{Is IME enabled?}}
B -->|Yes| C[Deliver event to IME] --> C1{{IME Response}}
B -->|No| F
C1 -->|Composed| D[Make RawKeyEvent from<br/> Composed text] --> RAW1
C1 -->|Composing| E[Render composing status]
C1 -->|Continue| F[Make RawKeyEvent] --> RAW1

RAW1{{match a phys: mapping?}}
RAW1 -->|Yes| RAWDONE1(( ))
RAW1 -->|No| RAW2{{match a raw: mapping?}}
RAW2 -->|Yes| RAWDONE1
RAW2 -->|No| RAW3{{match a mapped: mapping?}}
RAW3 -->|Yes| RAWDONE1
RAW3 -->|No| DEAD1{{Does RawKeyEvent complete a dead-key?}}

DEAD1 -->|Yes| I[Make KeyEvent from<br/>expanded dead key] --> KEY1
DEAD1 -->|No| DEAD2{{Does RawKeyEvent start a dead-key?}}
DEAD2 -->|No| J[Make KeyEvent from<br/>RawKeyEvent] --> KEY1
DEAD2 -->|Yes| DEADCOMP[Render composing status]

KEY1{{match a phys: mapping?}}
KEY1 -->|Yes| RAWDONE2(( ))
KEY1 -->|No| KEY2{{match a raw: mapping?}}
KEY2 -->|Yes| RAWDONE2
KEY2 -->|No| KEY3{{match a mapped: mapping?}}
KEY3 -->|Yes| RAWDONE2
KEY3 -->|No| M[Send key to terminal]

RAWDONE1 --> RAWDONE3[Perform assignment action]
RAWDONE2 --> RAWDONE3

Alt / Option Key Behavior & Composed Keys

The operating system has its own user selectable keymap that is sometimes at odds with old-school terminal emulation that pre-dates internationalization as a concept. WezTerm tries to behave reasonably by default, but also give you control in other situations.

Layouts with an AltGr key

If you have, for example, a European keyboard layout with an AltGr key then wezterm will respect the composition effects of AltGr produced by the system. For example, in a German keymap, AltGr < will produce |.

If your physical keyboard doesn't match the keyboard layout (eg: using a US keyboard with DEU selected in the OS), then the right hand Alt key is often re-interpreted as having the AltGr function with behavior as described above.

The left Alt will be treated as a modifier with no composition effects.

Microsoft Windows and Ctrl-Alt <-> AltGr

If you are using VNC and a keyboard layout with dead keys, then you may wish to enable treat_left_ctrlalt_as_altgr.

macOS Left and Right Option Key

since: 20200620-160318-e00b076c

The default behavior is to treat the left Option key as the Alt modifier with no composition effects, while the right Option key performs composition (making it approximately equivalent to AltGr on other operating systems).

You can control this behavior in your configuration:

return {
  send_composed_key_when_left_alt_is_pressed = false,
  send_composed_key_when_right_alt_is_pressed = true,
}

since: 20210203-095643-70a364eb

WezTerm is now able to perform dead-key expansion when use_ime = false. Dead keys are treated as composition effects, so with the default settings of send_composed_key_when_left_alt_is_pressed and send_composed_key_when_right_alt_is_pressed above, in a US layout, Left-Opt n will produce Alt N and Right-Opt n will will for a subsequent key press before generating an event; Right-Opt n SPACE will emit ~ whereas Right-Opt n n will emit ñ.

You may also set use_dead_keys = false to skip the hold state; continuing the example above, Right-Opt n will then immediately produce ~.

Input Method Editor (IME)

WezTerm has support for using the operating system Input Method Editor (IME) on some operating systems.

The use_ime docs have more information.

Dead Keys

since: 20201031-154415-9614e117

By default, if you are using a layout with dead keys (eg: US International layout, or a number of European layouts such as German or French) pressing a dead key in wezterm will "hold" the dead key until the next character is pressed, resulting in a combined character with a diacritic. For example, pressing ^ and then e will produce ê. Pressing ^ then SPACE will produce ^ on its own.

If you are a heavy user of Vi style editors then you may wish to disable dead key processing so that ^ can be used with a single keypress.

You can tell WezTerm to disable dead keys by setting this in your configuration file:

return {
  use_dead_keys = false,
}

Note that for X11 systems with use_ime=true, depending on the configured IME, the IME may handle dead key processing implicitly. There is no way for wezterm to prevent it from doing that, short of disabling the IME.

Defining Assignments for key combinations that may be composed

When a key combination produces a composed key result, wezterm will look up both the composed and uncomposed versions of the keypress in your key mappings. If either lookup matches your assignment, that will take precedence over the normal key processing.