wl-pick

A window switcher for wlroots compositors: a grid overlay of live window previews that looks like a rofi theme, and tells you which one you picked.

It replaces a wlthumbs | rofi pipeline, and doubles as a screencast source picker for the desktop portal. The difference is that no thumbnails exist: each window is captured straight into a wl_shm buffer that is handed to its own wl_subsurface, and wp_viewporter tells the compositor which rectangle to scale it into. There is no image encoding, no scaler, and no full-resolution bitmap in this process — which is also why it appears in about 60 ms and holds ~18 MB of RSS however many windows are open.

sway-tree       0.6ms     window list + con_ids over sway IPC
toplevels       0.2ms     ext-foreign-toplevel-list handles
constraints     1.5ms     every capture session's buffer size, in one roundtrip
capture        52.5ms     8 windows, all frames in flight at once
labels          0.0ms     shaped on a worker thread while the captures ran
mapped          4.9ms     layer surface + subsurfaces on screen

The capture phase is the compositor reading full-resolution window pixels out of the GPU. It is bandwidth-bound (~1.1 GB/s here) and unaffected by how large the thumbnails are — which also makes it a free window to do other work in. Loading a font and rasterising its first glyphs costs ~20ms, so labels are shaped on a worker thread started before the captures and joined after them, and cost nothing in wall clock.

Status

Working: a labelled grid of live previews with keyboard navigation. Type-to-filter is the one thing the rofi version had that this doesn't — see the roadmap.

Usage

wl-pick [--format tsv|json|portal] [--live all|current|none] [--fps N]
        [--no-outputs] [--hide-labels] [--font FAMILY] [--font-size PX]
        [--timeout SECS] [--verbose]

• --format tsv|json|portal how to report the pick (default tsv)
• --live all|current|none which tiles keep updating (default all; displays are always a single snapshot)
• --fps N cap on live updates per tile per second (default 12)
• --no-outputs windows only; displays are included as tiles by default
• --hide-labels draws an icon-only grid
• --font FAMILY label font family (default: the system monospace font)
• --font-size PX label size in logical px
• --config PATH config file (default ~/.config/wl-pick/config)
• --timeout SECS exits after a deadline, in case the keyboard grab ever traps you
• --verbose phase timings, the tile list, and capture stats

wl-pick is a chooser: the pick goes to stdout, nothing does if you cancel, and the exit status is 0 for a pick and 1 for a cancel. It never acts on the choice — it has no idea what you want to do with it. Focusing on sway looks like this:

#!/usr/bin/env bash
# ~/.local/bin/winmenu, bound to $mod+Tab
IFS=$'\t' read -r type id toplevel app title < <(wl-pick) || exit 0
case $type in
    window) swaymsg "[con_id=$id] focus" ;;
    output) swaymsg "focus output $id" ;;
esac

Windows only, as a one-liner:

swaymsg "[con_id=$(wl-pick --no-outputs | cut -f2)] focus"

Three formats, because the identifiers different consumers need differ:

portal is exactly what xdg-desktop-portal-wlr's simple chooser reads, so wl-pick can be the picker for getDisplayMedia and friends — with live previews of both windows and displays:

[screencast]
chooser_type=simple
chooser_cmd=wl-pick --format portal

Hovering deliberately does not move the selection — the keyboard keeps it, and a click acts on whatever is under the cursor. Clicking the margin, a gap, or an empty cell of a ragged last row does nothing. Tiles are subsurfaces, so a click on a thumbnail identifies its tile by surface; only clicks on the chrome around them need hit-testing.

Navigation reads raw evdev keycodes, so it is layout-independent — but it also means virtual-keyboard clients such as wtype (which invent their own keymap) cannot drive it. That goes away with xkb support, which filtering needs anyway.

Live previews

Capture sessions stay open, so a tile can be refreshed. Three things keep that from being expensive:

• It is damage-driven. After a session's first frame the compositor only produces another once the window content changes, so a request left outstanding on an idle window costs nothing. Measured over 4s with one animating window out of ten: 52,52,1,1,1,1,1,1,13,1 frames — the static windows delivered exactly their first frame and nothing more.
• Frame callbacks are the clock. Re-captures are driven by the overlay's own wl_surface.frame callbacks, so they stop when it isn't being presented, and --fps throttles per tile on top of that (12 fps measured as 12.1).
• Two buffers per window, alternating. A capture must not write into a buffer the compositor is reading, so each window gets two and wl_buffer.release decides which is free. Note that release is the entire contract: with wl_shm the compositor copies the pixels out at commit and hands the buffer straight back, so the slot on screen is usually free too — waiting for it to stop being displayed instead deadlocks after two frames.

The cost is memory and bandwidth: two full-resolution buffers per window (138 MB of shm for eight windows and a display here, against 83 MB with --live none) and a readback per refreshed frame. --live current refreshes only the selected tile, which is much cheaper and still reads as alive.

Config

~/.config/wl-pick/config, or --config PATH. Flat key = value lines with # comments, everything optional, and a flag always beats the file. No TOML dependency, because there is nothing to nest.

background     = #282828      # the grid's backdrop
foreground     = #ebdbb2      # label text
selection      = #d79921      # the highlighted tile
selection-text = #282828      # its label
border         = #d79921
border-width   = 2px

max-width      = 90ppt        # the box the grid may fill
max-height     = 90ppt
max-columns    = 4            # thumbnails are that box divided by these
max-rows       = 4

font           = monospace
font-size      = 13.3
labels         = yes
outputs        = yes
live           = all
fps            = 12
format         = tsv
timeout        = 0            # seconds; 0 means none

Sizes take sway's units: 600px is absolute, 90ppt a percentage — and the percentage resolves against the display the grid actually appears on, every time it runs. On a mixed setup one file gives 90% of a 1280-wide laptop panel and 90% of a 3840-wide monitor, rather than a pixel count that suits one and looks wrong on the other. The overlay maps explicitly on that display, at its scale, so mixed-DPI renders crisply either way.

All four sizing settings are caps:

• max-width and max-height bound the overlay.
• max-columns and max-rows bound the grid inside it.

A thumbnail is simply that box divided by those caps, which means its size never depends on how many windows are open: one window gets the same thumbnail as thirty, in a smaller overlay, because the overlay hugs whatever is actually there. Rows past max-rows scroll, with a scrollbar in the right margin, PgUp/PgDn, and the selection always kept in view. Tiles scrolled out of sight are unmapped, so live capture skips them too.

Turning labels off gives that row back to the thumbnails rather than shrinking the window, since the box is what you asked for either way.

Look

The defaults come from the rofi theme this replaces: gruvbox dark, a yellow selection filling the element padding, ceil(sqrt(n)) columns capped at 4, title · app centred underneath. Padding, gaps and margins are still fixed, in src/theme.rs.

The label font defaults to the system monospace font — whatever fc-match monospace answers, which is what the rest of the desktop uses. (cosmic-text's own generic resolves through a built-in preference that is usually not installed, and then lands on an arbitrary face, so it is asked directly instead; if fontconfig isn't available, a short list of common distribution defaults is tried.) --font names a family instead, and --verbose reports which family the labels were actually shaped with.

Naming a family scans your own font directories first because they are small; the full system scan (~37ms) happens only if it isn't found there. An unknown family falls back to whatever cosmic-text picks rather than failing. Long titles are ellipsised to the cell.

Requirements

A wlroots compositor advertising ext-image-copy-capture-v1, ext-image-capture-source-v1 (with the foreign-toplevel source manager), ext-foreign-toplevel-list-v1, wlr-layer-shell-unstable-v1 and wp_viewporter — sway 1.11+, and in principle Hyprland, labwc and jay, though only sway is tested. sway is also the source of truth for the window list, over its IPC socket, which is the one thing that would need replacing to run elsewhere (ext-foreign-toplevel-list-v1 already reports app id and title).

Known upstream issue: holding per-toplevel capture sessions open makes windows blurry on fractionally scaled outputs (sway#9113). Integer scales are unaffected. It matters more once previews are live.

Roadmap

• type-to-filter with fzf-quality fuzzy matching (and the xkb keyboard input it needs, which would also let virtual-keyboard clients drive the overlay)
• dmabuf capture, so the pixels never leave the GPU at all — and live previews stop costing a readback per frame

Source layout

main.rs     orchestration: list, capture, map, report the pick
cli.rs      flags, defaults, and the help text that documents them
sway.rs     the window list and display names, over sway's IPC socket
target.rs   what a tile stands for, and the three output formats
app.rs      the Wayland client state every event dispatches into
capture.rs  capture sessions, their buffers, and the live clock
overlay.rs  the layer surface, the drawing, and the keyboard
theme.rs    colours, grid geometry, aspect fitting
text.rs     label shaping on a worker thread
shm.rs      memfd allocation and the ARGB painter

Building

cargo build --release
cargo test          # grid geometry and hit-testing, ellipsising, output
                    # formats, glyph output