vterm/Architecture.md

vterm architecture

How the plugin is put together, for someone about to change it. README.md is the user-facing overview; this is the developer reference. Code style lives in CODE.md.

What it is

A terminal emulator that runs inside an ordinary Sublime Text tab. There is no custom drawing surface: the emulator's screen is written into the view as real text, and colours are region overlays. That single decision explains most of what follows.

Constraints it works under:

• Sublime Text 4, build 4149+ (View.folded_regions), plugin host Python 3.8 (.python-version). The host is a separate process, so every Sublime API call from plugin code is a cross-process round trip — call count is what costs.
• macOS only. The pty layer is posix and the mousemap is Default (OSX).
• Third-party code comes from Package Control libraries declared in dependencies.json: pyte (emulation), ptyprocess (pty), wcwidth (character widths).
• Editing plugin sources needs a Sublime restart: submodules are deliberately not purged on reload, because rebuilding the terminal registry would orphan open tabs.

Files

The package

File	Role
main.py	Entry point. Imports every command and listener class so Sublime discovers them, and defines plugin_loaded().
vterm/terminal.py	Terminal: pty process, emulator ownership, the reader/renderer threads, and everything written back to the app.
vterm/emulator.py	VtScreen and VtStream: pyte subclasses adding the terminal features these agents need.
vterm/render.py	Screen → view text, colour overlays, caret, folding, trimming.
vterm/scheme.py	SchemeManager: generates the colour scheme that defines the scopes the renderer paints with.
vterm/themes.py	The built-in palettes (default, homebrew, pro, slate).
vterm/commands.py	The Sublime commands: open, keypress, copy/paste, theme picker, clear, cleanup, close.
vterm/listeners.py	VtermCoreEventListener: focus, IME capture, copy/paste rerouting, the mousemap context.
vterm/mouse.py	Wheel and click handling, and the trackpad poller.
vterm/keys.py	Key name + modifiers → escape sequence tables.
vterm/utils.py	settings(), the responsive throttle, the intermission pacer, rev_wcwidth.

Sublime-facing files at the repo root

File	Role
vterm.sublime-settings	Defaults for the eleven plugin settings.
vterm.sublime-commands	Four palette entries: open, open terminal in project folder, clear screen, select theme.
vterm.sublime-syntax	An intentionally empty hidden syntax, so terminal text is never tokenized.
Default.sublime-keymap	245 generated bindings. Do not edit by hand — regenerate with tools/gen_keymap.py.
Default (OSX).sublime-mousemap	Two wheel bindings.
dependencies.json	Package Control libraries.

Generated at runtime, not in the repo: Packages/User/vterm/vterm.hidden-color-scheme.

Classes

• Terminal (terminal.py) — one per terminal tab. Owns the pty, the VtScreen and VtStream, and the two threads. Instances live in the class-level registry Terminal._terminals, keyed by view id; Terminal.from_id(view.id()) is how every command and listener finds its terminal, and it is a plain dict lookup, so it is the cheapest possible gate to put at the top of a hook.
• TerminalPtyProcess (terminal.py) — ptyprocess.PtyProcess that accepts str.
• VtScreen (emulator.py) — pyte.Screen plus the alternate screen, scrollback, synchronized output, mouse/focus/paste modes, cursor styles, query replies, OSC handling. Mutated only by the renderer thread, so it carries no lock.
• VtStream (emulator.py) — pyte.Stream with a rewritten parser FSM. Stock pyte drops what these agents rely on: >/</= prefixed CSI, the DECRQM $p intermediate, and OSC codes longer than one digit. Its feed() regex-matches complete plain CSI and dispatches them directly (params memoized) — pyte resumes the parser coroutine once per character, which dominated escape-heavy floods. OSC, charsets, private/prefixed CSI and : subparams still take the coroutine, byte for byte; a read ending mid-CSI is carried into the next one. Parity is enforced by a differential test (run_fast_path_cases).
• Cursor (emulator.py) — replaces pyte's so the default attribute set can be a Char carrying the extra linefeed field.
• SchemeManager (scheme.py) — module singleton scheme_manager. Owns the generated scheme file and the mapping from colour combination to scope name.
• VtermRenderCommand (render.py) — Sublime keeps one command instance per view, so its colored_lines, _row_cache, _key_counter, the scrollback chunk bookkeeping (_archived_chunks, _open_rows, _open_chunk, _trimmed) and _lastrow are that view's render state.

Commands

Command	Class	Reached from
vterm_open	VtermOpenCommand	palette, user keybindings
vterm_open_terminal_project_folder	VtermOpenTerminalProjectFolderCommand	palette
vterm_initialize_view	VtermInitializeViewCommand	internal
vterm_keypress	VtermKeypressCommand	244 keymap bindings
vterm_render	VtermRenderCommand	renderer thread
vterm_show_cursor	VtermShowCursorCommand	internal
vterm_copy / vterm_paste / vterm_paste_text	commands.py	on_text_command, IME capture
vterm_select_theme	VtermSelectThemeCommand	palette
vterm_clear_screen	VtermClearScreenCommand	palette, super+shift+k
vterm_cleanup	VtermCleanupCommand	renderer, on process exit
vterm_close	VtermCloseCommand	nothing — available for a user keybinding
vterm_clear_undo_stack / vterm_noop	commands.py	internal
vterm_scroll / vterm_mouse_click	mouse.py	mousemap, drag_select interception

Two application-wide EventListeners: VtermCoreEventListener (listeners.py) and VtermMouseEventListener (mouse.py).

Data flows

Opening a terminal

vterm_open → resolve cmd (default $SHELL -l) and cwd (first project folder → active file's directory → ~) → scheme_manager.initialize() → window.new_file() → vterm_initialize_view stamps the view settings → Terminal(view) → Terminal.start(): register in _terminals, compute the size from the viewport, spawn the pty, build the VtScreen/VtStream, subscribe to palette changes, start the two threads. All on the UI thread; a failure closes the view again.

Output: a pty byte becomes a visible character

child writes
   │
   ├─ reader thread ──────────────────────────────────────────────
   │   process.read(8192)          blocking read on the pty master
   │   self._decoder.decode(raw)   incremental UTF-8: a sequence split
   │                               across reads must not be corrupted
   │   chunks.append(text)         collections.deque, no lock
   │
   ├─ renderer thread, every 30 ms (RENDER_PERIOD) ───────────────
   │   feed_data()                 popleft up to FEED_LIMIT (256 KiB)
   │   stream.feed(joined)         → VtStream parser → VtScreen.draw
   │                                 mutates buffer, fills screen.dirty
   │   in_sync_frame()?            mid ?2026 frame → skip this tick
   │   is_visible()?               hidden tab → draw nothing this tick
   │   was_resized()?              (checked once a second) → resize pty
   │   is_render_due()?           unfocused → at most one frame per
   │                               UNFOCUSED_RENDER_PERIOD
   │   _need_to_render()?          dirty rows, a pending clear, or a
   │                               moved cursor
   │   view.run_command("vterm_render")
   │   _sync_wheel_flag()          publishes the mousemap gate, and starts the
   │                               trackpad poller while the app owns the wheel
   │
   └─ UI thread ──────────────────────────────────────────────────
       VtermRenderCommand.run(edit)
         update_lines()   → drain screen.history above `offset`,
                            then each dirty screen row
           update_line()  → segment_buffer_line() → row-cache check
                            → view.replace() + add_regions()
         sync_alt_fold()
         follow-scroll gate → trim, focus_cursor()*, request_scroll()
         sync_cursor_style(), set_name()*, dirty.clear()
                            * focused terminals only

Rendering off focus. A terminal only touches its view the way the user would while it holds focus. Sublime dispatches view events application-wide, and packages hang global state off them: Vintage's InputStateTracker.on_selection_modified resets one module-level g_input_state for every view in every window, so a caret this plugin moved at 33 fps erased the half-typed d of a dd in whatever file the user was editing. Sublime drops a pending key sequence over the same kind of traffic, which took gg and ciw with it.

Three states, from Terminal.focused (set by on_activated / on_deactivated) and Terminal.is_visible() (polled once a second, because selecting another tab in an unfocused group raises no event for this view):

state	text	caret, sel	tab name	follow-scroll	draw rate
focused	yes	yes	yes	yes	33 fps
visible, unfocused	yes	no	no	yes	15 fps
hidden	no	no	no	no	none

A frame costs a command dispatch and a dozen UI round trips whatever changed — two rowcol(size()), folded_regions(), viewport_position(), the trim_trailing_spaces scan, then layout_extent() / text_to_layout() / set_viewport_position() — so the rate a terminal is drawn at is what the editor pays for it, and only a focused one is worth 33 fps.

The renderer tick itself stays at RENDER_PERIOD in every state: feed_data() runs inside it, and throttling that would cap the parser at FEED_LIMIT per tick while the reader keeps filling an unbounded chunks. Only the drawing is rate-limited. So the emulator is always current and one render on activation catches the view up. An unfocused terminal is left with an empty selection — nothing a render edit can shift, so no selection event of ours escapes — unless the user selected text to copy, which is kept.

The view/offset model. The view holds real text. Rows 0 .. offset-1 are scrollback; rows offset .. offset + screen.lines - 1 are the live screen. terminal.offset is the view row of screen row 0. When the emulator retires lines into screen.history, the renderer drains them, advances offset by that many rows, and writes them at their now permanent view rows. Once a line leaves screen.history, its only copy is the view text — which is why the alternate screen folds the scrollback rather than deleting it. A fresh view is given one blank scrollback row before anything else: Sublime lays out nothing above row 0, and the trackpad poller needs that slack to see an upward gesture at all — on the alt screen no line ever retires to provide it.

The row cache. pyte's dirty set is a repaint hint, not a diff: scrolling one line marks every row dirty, and a TUI repaints whole frames. update_line compares (segments, lf) against _row_cache and returns early when unchanged, which is what keeps a full-screen repaint from costing hundreds of cross-process calls.

The follow-scroll gate. following() compares the viewport against terminal.viewport_y, where the last auto-scroll left it. Above that, the user scrolled up by hand, and trimming, caret placement and scrolling are all skipped so their position survives the next frame. scroll_view (the PgUp/PgDn and Up/Down chords) closes the gate the same way, and scrolling down clamps to the anchor bottom_y returns, which re-opens it. A one-row scroll lands exactly on the tolerance boundary, so one Up closes the gate and one Down re-opens it.

Pinning waits for rest. Two pollers put the viewport back on the anchor: the renderer tick, for a viewport dragged into sublime's past-end room on a terminal too idle to draw a frame, and the trackpad poller, when a browsing gesture returns to the anchor. Both compare the viewport against the position they saw one tick earlier and act only when the two agree. macOS keeps scrolling after the fingers lift, and a hard set_viewport_position against that momentum used to yank the view back thirty times a second for as long as the glide lasted — a visible shake at the bottom of every scroll.

How colour is actually drawn

view.add_regions(key, regions, scope) can only name a scope. It cannot take a colour. The colours must already exist in the view's colour scheme. So vterm generates the colour scheme at runtime:

1. segment_buffer_line splits a screen row into runs of identical attributes.
2. For each non-default run, scheme_manager.scope(fg, bg, style) returns a scope name such as vterm.red.default.b. On a miss it creates the rule and schedules a write; the hot path is a single lock-free dict read.
3. colorize_line calls add_regions once per distinct scope in the row, with keys vterm#N — monotonic rather than per-row, because trim_history renumbers rows and per-row keys would collide. colored_lines[row] remembers them so they can be erased. Rows that scroll off the screen are queued by archive_rows and merged by flush_chunk onto shared vterm@chunk@scope keys (one per scope per CHUNK rows of scrollback), their per-row keys erased — every key is a separate pass in Sublime's redraw, and a long coloured session used to accumulate ~15k of them. The merge waits for the chunk to close: add_regions replaces a key's whole set, so merging every frame re-sent everything the chunk held so far, which is quadratic in CHUNK when output arrives a few rows at a time. The open chunk's rows keep their per-row keys until then, so the live count is scopes x chunks plus at most one chunk's worth. Chunk ordinals are view row + _trimmed, so trim_history renumbering cannot move a row between chunks; trimming settles the open chunk first, then erases whole chunks' keys and compacts the boundary chunk.
4. _write_file serializes every rule into Packages/User/vterm/vterm.hidden-color-scheme, the path the view's color_scheme setting points at. Each write makes Sublime reload the scheme and restyle every view, so writes are debounced 250 ms onto the async thread.

Details worth knowing before touching scheme.py:

• Rules are seeded at build time for all 16 foregrounds (plain and bold) so the very first frame never references a scope the file lacks.
• If a rule's resolved background equals the global background exactly, it is nudged by one (next_color) — add_regions inverts foreground and background when they match.
• Colours are resolved when a rule is added, so a palette change cannot append: _build rebuilds the whole table.
• Truecolor SGR is quantized into the 256 palette (quantize_256), which bounds the rule table: every distinct pair would otherwise be a permanent rule and another file write.
• plugin_loaded writes the file synchronously, because a restored tab's color_scheme is read during startup.

The alternate screen

While an app owns the alternate buffer, sync_alt_fold folds rows 0..offset-1 away, so the view shows the app's screen alone. The scroll range is deliberately left in place: the trackpad poller pins the viewport and needs the slack to read a gesture. The fold is re-checked every frame, not just on transitions, because the screen's first row is rewritten on every region scroll and an edit at the fold boundary can silently break a fold. Entering the alt screen collapses the layout, so that transition also zeroes terminal.viewport_y; a stale anchor would hold the follow-scroll gate shut. A fold merely repaired keeps the anchor, or the boundary row apps rewrite on every scroll would thrash the poller. The emulator side (switch_to_screen) swaps in a maxlen=0 history, so a full-screen app never pushes its frames into scrollback.

Input: a key press reaches the app

Almost every key, including every printable character, is bound to vterm_keypress. That is deliberate: if printables were left to Sublime, an app that does not echo (vim in normal mode) would leave the typed character sitting in the view.

keymap binding (context: setting.vterm_view)
  → VtermKeypressCommand.run(key=…, ctrl=…, alt=…, shift=…)
  → view_scrolled()          with scroll_modifier, PgUp/PgDn scroll the view a page
                             and Up/Down a row; stops here, never on the alt screen
  → Terminal.send_key()      adds application_mode (DECCKM), new_line_mode (LNM)
  → keys.get_key_code()      modified nav keys → CSI 1;<mod><final>
                             enter/tab/backspace/space, modified, while the app
                             asked for modifyOtherKeys → CSI 27;<mod>;<code>~
                             ctrl → control byte; alt → ESC prefix
                             plain → _KEY_MAP / _APP_MODE_KEY_MAP
  → Terminal.send_string(normalized=False)
  → pty write                ≤4096 bytes inline, larger goes through a queue

IME and composed input cannot work this way — there is no key event for "the IME committed 日本語"; the text arrives as Sublime's own insert command. on_modified catches that: it reconstructs what was inserted from the caret positions (not from args["characters"], which reports the typed keys rather than the committed composition), forwards it as an unbracketed paste, and soft_undos any other command that modified the view. Text the emulator did not produce must never remain in the view.

Copy and paste are rerouted by on_text_command: copy → vterm_copy (which strips the zero-width CONTINUATION sentinel so soft-wrapped lines re-join), paste → vterm_paste, and undo → vterm_noop. Pastes are wrapped in ESC[200~ / ESC[201~ when the app enabled bracketed paste. In the other direction, OSC 52 from the app reaches the system clipboard via a hop to the UI thread.

Mouse

Once per tick, _sync_wheel_flag publishes vterm_view.wheel_captured = "the app reports the mouse, or owns the alternate screen". The mousemap's binding is gated on it, which produces three cases:

1. App tracks the mouse → vterm_scroll maps the event to a screen cell and sends three SGR wheel reports, one per write — pi matches the SGR form against a whole stdin chunk and drops anything longer.
2. Alternate screen, no tracking → three arrow keys per notch (alternate scroll, as kitty/iTerm2/xterm do). This is what scrolls a fullscreen UI before it enables mouse reporting.
3. Neither → the binding does not match at all and Sublime scrolls the view over the real scrollback text.

The trackpad fires none of that: on macOS it never reaches a mousemap binding (sublime_text#1838), and its only trace is the viewport moving. So while the wheel flag is up, start_trackpad runs a 24 ms timer chain, and any displacement from the follow anchor is a gesture — whole lines of it go out through the same send_scroll as the wheel, the fraction is carried into the next tick, and the viewport is pinned back to the anchor. A displacement of a viewport or more is a scrollbar drag or a page scroll instead: the poller disengages and lets the user browse until they return to the anchor. _sync_wheel_flag restarts the chain within a frame or two of it stopping on a hidden tab or a capture flap, and scroll_to_cursor yields while the poller owns the pin — apps repaint mid-gesture, and re-pinning there would discard displacement not yet harvested.

Clicks: VtermMouseEventListener intercepts drag_select only when the app tracks the mouse, and only the plain single-button form, so shift-click and add-selection stay native. The first click on an unfocused tab only focuses it — we replaced the command that normally does that — and subsequent clicks send a press/release pair.

Lifecycle

• Tab closed → on_pre_close → Terminal.kill(): unsubscribe, terminate the child, drop the registry entry.
• Process exits → reader gets EOFError, sets alive = False; the renderer drains the last output, then posts vterm_cleanup to the UI thread. Cleanup latches vterm_view.finished, renders a final frame, kills the terminal, and either closes the tab (auto_close) or appends [process exited with code N].
• Plugin host restart → the registry is gone but the view survives with its settings and text. on_activated detects a view marked vterm_view with no terminal behind it and shows a "disconnected" status: the tab has to be closed and reopened.

Threading

Thread	Runs	Touches
UI	commands, listeners, Terminal.start, scroll_to_cursor, _trackpad_tick, clipboard	Sublime API; small pty writes go out here too
reader	process.read, decode, chunks.append	no Sublime API, no screen state
renderer	stream.feed, then run_command("vterm_render")	owns the emulator
paste drain	transient; empties the 4096-byte chunk queue	pty only
ST async worker	SchemeManager._flush	the scheme file

The emulator needs no lock because exactly one thread mutates it. The chunk deque needs none either: append and popleft are atomic under the GIL. This matters — the renderer used to hold a lock across stream.feed and the whole render command, so the reader could not drain the pty for the duration of a frame and the child blocked in write(). Backpressure now lives in FEED_LIMIT instead: at most 256 KiB is parsed per tick, so a huge burst still renders intermediate frames while the reader keeps draining.

SchemeManager does keep a lock, because scope() is called from the renderer while reload() and _flush() run on other threads. Its read fast path skips the lock; the invariant that makes that safe is that _names must never outlive rules — _build replaces both together.

Sublime settings

Plugin settings (vterm.sublime-settings, read through the cached settings() handle): auto_close, scrollback_history_size, honor_erase_scrollback, scroll_modifier, min_columns, max_columns, theme, ansi_colors, terminal, parameters, debug. scroll_modifier is the modifier that turns PgUp/PgDn and Up/Down into a scroll of the view — a page, a row — instead of a key for the app. terminal and parameters name the external terminal spawned by vterm: Open Terminal in Project Folder (%CWD% in parameters becomes the project folder). ED 3 never touches the visible screen (as in xterm), and honor_erase_scrollback is off, so it does not drop the scrollback either: pi and Claude Code repaint by erasing saved lines, which would take the history with them (cmd+shift+k still empties the tab). Changing theme fires SchemeManager.reload, which rebuilds the palette and restyles open terminals in place.

Stamped on every terminal view by vterm_initialize_view: vterm_view (the marker every context and hook keys off), is_widget, color_scheme (the generated scheme), syntax (the empty one), scroll_past_end, scroll_context_lines, and a list of disables — gutter, highlight_line, word_wrap, auto_complete, draw_white_space, draw_indent_guides, spell_check, translate_tabs_to_spaces, command_mode (which defuses Vintage mode inside a terminal). The view is also set scratch, so closing it never prompts to save.

Written at runtime: vterm_view.finished (latched by cleanup; also disables the Escape binding for a dead tab), vterm_view.wheel_captured (the mousemap gate), and block_caret / caret_style (from DECSCUSR; style 0 is the terminal default, a blinking block).

Contexts. Keymap bindings use the native setting.vterm_view, evaluated in Sublime's own process — a plugin on_query_context would cost a round trip on every keystroke in every view. Mousemaps do not evaluate setting. contexts the same way, so the single wheel context still goes through on_query_context, which answers that one key and returns None for everything else.

---

Terminal concepts, briefly

For anyone who has not worked with terminal protocols before. The ones vterm implements are marked ✅.

Control bytes — single characters below 0x20 with an effect rather than a glyph: CR (\r) return to column 0, LF (\n) next line, BS backspace, HT tab, BEL bell.

Escape sequence — ESC (0x1B) followed by more bytes. Everything below is one.

• CSI ✅ — Control Sequence Introducer, ESC [. The workhorse: ESC[2J clear screen, ESC[10;5H move the cursor. Ends with a final byte that names the operation; numbers in between are its parameters.
• SGR ✅ — Select Graphic Rendition, CSI … m. Sets colour and style: ESC[1m bold, ESC[31m red foreground, ESC[38;5;208m a 256-palette colour, ESC[38;2;r;g;bm truecolor, ESC[0m reset. This is what TUIs emit most.
• OSC ✅ — Operating System Command, ESC ], ending at BEL or ST. Carries strings rather than numbers: OSC 0/2 set the window title, OSC 10/11 query the default foreground/background, OSC 52 reads or writes the system clipboard.
• DCS / APC — Device Control String (ESC P) and Application Program Command (ESC _). Long payloads: sixel images, kitty graphics. vterm parses their framing and discards the payload, so an image never leaks into the text.
• SS3 ✅ — ESC O, a single shifted character. Cursor keys take this form in application mode; F1-F4 use it too.
• ST — String Terminator, ESC \, which ends OSC/DCS/APC payloads (BEL also ends OSC).

Modes are switches. DEC private modes are set with CSI ? n h (DECSET) and cleared with CSI ? n l (DECRST); DECRQM ✅ (CSI ? n $p) asks whether one is on, which is how an app discovers what the terminal supports.

• 1049 ✅ alternate screen — a second, scrollback-less buffer for full-screen apps, so leaving vim restores your shell exactly as it was.
• 2026 ✅ synchronized output — "I am mid-frame, do not draw yet"; what makes a fullscreen UI update without tearing.
• 2004 ✅ bracketed paste — pasted text is wrapped in ESC[200~/ESC[201~ so an app can tell it apart from typing and not auto-indent it.
• 1000 / 1002 / 1003 ✅ mouse reporting — clicks / clicks with drag / all motion.
• 1006 ✅ SGR mouse encoding — the modern wire form, ESC[<button;col;rowM, without the 223-column limit of the original.
• 1004 ✅ focus reporting — ESC[I on focus, ESC[O on blur.
• DECCKM ✅ — application cursor keys: arrows switch from CSI A to SS3 A.
• DECAWM ✅ — autowrap at the right margin.
• DECSCNM ✅ — reverse video for the whole screen.
• DECSCUSR ✅ (CSI n SP q) — caret shape: block, underline or bar, blinking or not.
• DECSTBM ✅ (CSI t;b r) — set the scrolling region to a band of rows.
• DECKPAM — application keypad mode, the numeric keypad's equivalent of DECCKM. vterm ignores it.
• LNM ✅ — line feed/new line mode; decides whether Enter sends CR or CR LF.

Queries — the app asks, the terminal answers on the input stream, as if typed:

• DA1 / DA2 ✅ (CSI c, CSI > c) — Device Attributes: what kind of terminal are you, and what version. Claude Code waits on DA1 with no timeout, so failing to answer hangs it.
• DSR / CPR ✅ (CSI n, CSI 6n) — Device Status Report / Cursor Position Report: "are you ok" and "where is the cursor". Private DSR 996 reports whether the palette is light or dark.
• XTVERSION ✅ (CSI > q) — the terminal's name and version. Naming ourselves is what lets apps enable 2026 on a generic TERM.
• Kitty keyboard protocol — a richer key encoding with release events. vterm answers the probe (CSI ? u) with zero flags, so apps fall back to something else — for pi that is modifyOtherKeys, which it then enables unprompted.
• modifyOtherKeys ✅ (CSI > 4 ; n m) — xterm's pre-kitty answer to the same problem: keys whose modified form has no legacy encoding are sent as CSI 27 ; <mod> ; <code> ~. While an app has it on, vterm uses that form for modified enter, tab, backspace and space — Shift+Enter is CSI 27;2;13~, since a bare ESC CR is indistinguishable from Alt+Enter. Ctrl+letters keep their control bytes, so Ctrl+C stays \x03.

Other terms

• pty — pseudo-terminal: a kernel device pair. The child thinks its stdin/stdout is a terminal; we hold the master end and read what it writes.
• TIOCSWINSZ / SIGWINCH ✅ — telling the pty its new size delivers SIGWINCH to the child, which is how a resize reaches the app.
• wcwidth ✅ — how many columns a character occupies: 1 for ASCII, 2 for CJK and most emoji, 0 for combining marks. Terminals are grids, so this has to agree with the font or the caret drifts.
• IME — Input Method Editor: the OS layer that turns keystrokes into composed text (pinyin → 中文, dead keys → accents). It commits text without a matching key event, which is why on_modified exists.
• TERM / COLORTERM ✅ — environment variables describing the terminal's capabilities. Children get TERM=xterm-256color and COLORTERM=truecolor.