feat(app): wire the app icon into the GUI, waybar, and SDL window

screencast_icon/ holds the single source icon set; meson install ships
the 256 px PNG into the hicolor theme for the GTK panel (themed icon
name) and the waybar CSS, and a custom target embeds it as
icon_png_data.h so the SDL receiver window sets it via SDL_SetWindowIcon
with an optional SDL3_image dependency (built-in find_library, since no
pkg-config file ships with it).

.agents/MEMORY.md

@@ -65,6 +65,23 @@ loopback; current phase is Phase 7.
   before the fix). Cross-compiling on the dev machine was considered and
   dropped — the on-Pi build works and the toolchain/container effort was
   not needed.
+- **App icons wired up** (`screencast_icon/`): single 256 px PNG used
+  everywhere. `meson install` ships it to
+  `share/icons/hicolor/256x256/apps/screencast.png`; the GTK panel sets it
+  via `set_icon_name("screencast")` (GTK4 removed the pixel-buffer window
+  icon — themed names only; `gtk_icon_theme_has_icon` verified OK); the
+  SDL receiver window embeds the PNG at build time (`icon_png_data.h` via
+  `scripts/icon_to_header.py` custom_target) and `SDL_SetWindowIcon` with
+  an optional SDL3_image dependency (no pkg-config ships with it →
+  `meson.get_compiler('cpp').find_library('SDL3_image', required: false)`
+  + disabler; `-DSC_HAS_WINDOW_ICON` gate; Pi builds without it still
+  work). The waybar widget now shows the icon as `background-image` in
+  `~/.config/waybar/style.css` (▶/⏸ text hidden; `.idle` dimmed,
+  `.streaming` green tint) instead of text glyphs; backup
+  `style.css.bak-20260909-*`. Lesson: icons appear in compositor
+  taskbars/switchers, not title bars (GTK4 CSD and Hyprland decorations
+  don't draw them), so validate via theme lookup, not title-bar
+  screenshots. `sudo pacman -S sdl3_image` done (extra/sdl3_image 3.4.6).
 - **GTK panel + waybar widget**: `screencast-gui` (gtkmm-4.0, behind
   `-Dgui=true`; app internals now live in the `sc_app_core` static lib so
   CLI and GUI share pipelines/session/state). `screencast waybar [--toggle]`

README.md

@@ -76,10 +76,13 @@ screencast waybar       # one JSON line for a waybar custom module
 screencast waybar --toggle
 ```
 
-The waybar module shows `▶` while streaming (tooltip: receiver, bitrate,
-elapsed) and `⏸` while idle; left-click toggles streaming to the last
-receiver, right-click opens the panel. All front-ends agree on state because
-the sender publishes it to `$XDG_RUNTIME_DIR/screencast/sender.json`.
+The waybar module shows the app icon (dimmed while idle, highlighted while
+streaming; tooltip: receiver, bitrate, elapsed); left-click toggles
+streaming to the last receiver, right-click opens the panel. All front-ends
+agree on state because the sender publishes it to
+`$XDG_RUNTIME_DIR/screencast/sender.json`. The icon (from `screencast_icon/`)
+is installed into the hicolor theme by `meson install`, used by the GTK
+panel and the waybar CSS, and embedded in the SDL receiver window.
 
 ## Under the hood: resilience
 

docs/RUNBOOK.md

@@ -146,11 +146,24 @@ screencast waybar --toggle   # stop a running sender / restart the last
 ```
 
 The waybar module (installed to this machine's `~/.config/waybar/` — see
-`custom/screencast` there): `▶` while streaming (tooltip: receiver, bitrate,
-elapsed), `⏸` while idle; left-click toggles streaming to the last receiver
-via the state file, right-click opens the panel. The sender publishes its
-state to `$XDG_RUNTIME_DIR/screencast/sender.json`, so every front-end —
-CLI, GUI, widget — agrees on what is running.
+`custom/screencast` there): the app icon as the module face (dimmed while
+idle, green tint while streaming; the `▶`/`⏸` text from the JSON is hidden
+in the CSS), tooltip with receiver, bitrate, elapsed; left-click toggles
+streaming to the last receiver via the state file, right-click opens the
+panel. The sender publishes its state to `$XDG_RUNTIME_DIR/screencast/sender.json`,
+so every front-end — CLI, GUI, widget — agrees on what is running.
+
+**Icons**: `screencast_icon/screencast_256.png` is the single app icon.
+`meson install` puts it in `share/icons/hicolor/256x256/apps/screencast.png`;
+the GTK panel uses it by themed name (`set_icon_name` — GTK4 removed the
+pixel-buffer window icon, so themed icons only), and the waybar CSS
+references the installed path. The SDL receiver window embeds the PNG at
+build time (`icon_png_data.h` generated by `scripts/icon_to_header.py`) and
+sets it via `SDL_SetWindowIcon` when SDL3_image is available (optional
+dependency; no pkg-config ships with it, so meson finds the library
+directly). Icons surface in compositor taskbars/switchers, not in title
+bars (neither GTK4's CSD nor Hyprland's decorations draw them).
+Restart waybar after changing `style.css` to pick up icon changes.
 
 ## Under the hood: resilience (Phase 7)
 

meson.build

@@ -26,6 +26,12 @@ if enable_tests
     subdir('tests')
 endif
 
+# Install the application icon into the hicolor theme: the waybar widget CSS
+# and any future .desktop file reference it.
+install_data('screencast_icon/screencast_256.png',
+    install_dir : get_option('datadir') / 'icons/hicolor/256x256/apps',
+    rename : 'screencast.png')
+
 # Summary for the user
 summary({
     'tests': enable_tests,

scripts/icon_to_header.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+"""Generate a C++ header that embeds an image file as a byte array.
+
+Usage: icon_to_header.py INPUT_IMAGE OUTPUT_HEADER
+"""
+
+import sys
+
+
+def main() -> int:
+    if len(sys.argv) != 3:
+        print(f"usage: {sys.argv[0]} INPUT_IMAGE OUTPUT_HEADER", file=sys.stderr)
+        return 2
+
+    with open(sys.argv[1], "rb") as src:
+        data = src.read()
+
+    words = ", ".join(f"0x{byte:02x}" for byte in data)
+    header = f"""// GENERATED FILE - do not edit by hand.
+// Produced by scripts/icon_to_header.py from screencast_icon/screencast_256.png.
+
+#pragma once
+
+#include <array>
+#include <cstdint>
+
+namespace sc {{
+inline constexpr std::array<std::uint8_t, {len(data)}> app_icon_png = {{
+    {words}
+}};
+}}  // namespace sc
+"""
+    with open(sys.argv[2], "w") as dst:
+        dst.write(header)
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())

src/gui/gui.cpp

@@ -59,6 +59,9 @@ class SenderWindow : public Gtk::ApplicationWindow {
     SenderWindow() {
         set_title("screencast");
         set_default_size(440, 440);
+        // GTK4 only supports themed icons: the PNG is installed into the
+        // hicolor theme (see meson.build) and picked up by name.
+        set_icon_name("screencast");
 
         auto* box = Gtk::make_managed<Gtk::Box>(Gtk::Orientation::VERTICAL, 8);
         set_child(*box);

src/meson.build

@@ -1,6 +1,16 @@
 # Core public headers / include dependency.
 sc_core_inc = include_directories('../include')
 
+# Embed the application icon so the GUI window and the SDL window icon work
+# from the build tree, after install, and on headless targets without any
+# runtime path lookup.
+icon_to_header = find_program('../scripts/icon_to_header.py')
+icon_png_data = custom_target('icon_png_data',
+    input : files('../screencast_icon/screencast_256.png'),
+    output : 'icon_png_data.h',
+    command : [icon_to_header, '@INPUT@', '@OUTPUT@'])
+sc_icon_dep = declare_dependency(sources : [icon_png_data])
+
 # The sender needs the PipeWire / xdg-desktop-portal capture backend;
 # receiver-only builds skip it entirely.
 build_sender = get_option('sender')

src/render/meson.build

@@ -2,14 +2,30 @@
 
 dep_sdl3 = dependency('sdl3')
 
+# SDL3_image (window icon) is optional: no pkg-config file ships with it,
+# so find the library directly. Builds without it (e.g. the Pi receiver)
+# simply skip the window icon.
+cc = meson.get_compiler('cpp')
+dep_sdl3_image = cc.find_library('SDL3_image', required: false)
+if not dep_sdl3_image.found()
+    dep_sdl3_image = disabler()
+endif
+
+sc_render_args = []
+if dep_sdl3_image.found() and cc.has_header('SDL3_image/SDL_image.h', dependencies : dep_sdl3_image)
+    sc_render_args += ['-DSC_HAS_WINDOW_ICON=1']
+endif
+
 sc_render_sources = files('sdl_renderer.cpp')
 
 sc_render = static_library('sc_render',
     sc_render_sources,
     include_directories : sc_core_inc,
-    dependencies : [dep_sdl3])
+    cpp_args : sc_render_args,
+    dependencies : [dep_sdl3, dep_sdl3_image, sc_icon_dep])
 
 sc_render_dep = declare_dependency(
     link_with : sc_render,
     include_directories : sc_core_inc,
-    dependencies : [dep_sdl3])
+    compile_args : sc_render_args,
+    dependencies : [dep_sdl3, dep_sdl3_image])

src/render/sdl_renderer.cpp

@@ -4,6 +4,16 @@
 
 #include <string>
 
+#ifdef SC_HAS_WINDOW_ICON
+
+#include <SDL3_image/SDL_image.h>
+
+#include <cstdio>
+
+#include "icon_png_data.h"
+
+#endif
+
 namespace sc {
 namespace {
 
@@ -52,6 +62,10 @@ class SdlRenderer final : public Renderer {
             (void)SDL_HideCursor();
         }
 
+#ifdef SC_HAS_WINDOW_ICON
+        set_window_icon();
+#endif
+
         renderer_ = SDL_CreateRenderer(window_, nullptr);
         if (renderer_ == nullptr) {
             last_error_ = std::string{"SDL_CreateRenderer failed: "} + SDL_GetError();
@@ -155,6 +169,29 @@ class SdlRenderer final : public Renderer {
     }
 
   private:
+#ifdef SC_HAS_WINDOW_ICON
+    // The icon is embedded at build time (see icon_png_data.h), so it works
+    // without any runtime file lookup. Harmless under KMSDRM, where there is
+    // no window manager to display it.
+    void set_window_icon() {
+        auto* stream = SDL_IOFromConstMem(sc::app_icon_png.data(), sc::app_icon_png.size());
+        if (stream == nullptr) {
+            std::fprintf(stderr, "screencast: failed to create icon stream: %s\n", SDL_GetError());
+            return;
+        }
+        // closeio=true: SDL_image consumes the stream, success or failure.
+        SDL_Surface* icon_surface = IMG_Load_IO(stream, true);
+        if (icon_surface == nullptr) {
+            std::fprintf(stderr, "screencast: failed to load window icon: %s\n", SDL_GetError());
+            return;
+        }
+        if (SDL_SetWindowIcon(window_, icon_surface) != 0) {
+            std::fprintf(stderr, "screencast: failed to set window icon: %s\n", SDL_GetError());
+        }
+        SDL_DestroySurface(icon_surface);
+    }
+#endif
+
     bool recreate_texture(int width, int height) {
         if (texture_ != nullptr) {
             SDL_DestroyTexture(texture_);