Add support of virtual monitor

CHANGELOG.md

@@ -8,6 +8,14 @@ daemon ship together under one version.
 
 ### Added
 
+- Cast a **new monitor** instead of mirroring one you already have. A toggle
+  above the device list switches between *Mirror screen* and *New monitor*; in
+  the second mode the desktop gains an extra display, sized to what the receiver
+  accepts, that exists only for as long as the cast. The television becomes a
+  real second screen you can drag windows onto rather than a copy of your laptop
+  panel, and it is no longer letterboxed to fit it. Needs a Wayland session -
+  GNOME on Xorg cannot create a monitor, so the toggle is hidden there.
+  *Choose what to cast* also lists GNOME's own Virtual Monitor option now.
 - Eleven more interface languages: Arabic, Bengali, Bulgarian, Chinese
   (Simplified), French, Hindi, Indonesian, Portuguese (Brazil), Russian,
   Spanish and Urdu.

README.md

@@ -1,6 +1,6 @@
 # GNOME Shell Cast
 
-Cast your **whole screen or a single window** to a **Chromecast** device, or send your **system audio** to a Chromecast speaker, smart display, or cast group, right from the GNOME Shell top panel.
+Cast your **whole screen or a single window** to a **Chromecast** device, add a **new monitor** and cast that instead, or send your **system audio** to a Chromecast speaker, smart display, or cast group, right from the GNOME Shell top panel.
 
 The project has two parts, shipped together in this repository:
 
@@ -29,7 +29,7 @@ The two talk over the D-Bus session bus (`org.gnome.ShellCast1`). The daemon is
 
 Chromecast's Default Media Receiver plays media it pulls over HTTP. When you start a cast:
 
-1. The daemon opens an XDG ScreenCast portal session - GNOME shows its native picker for a monitor or a window.
+1. The daemon opens an XDG ScreenCast portal session - GNOME shows its native picker for a monitor or a window. In *New monitor* mode it asks the portal for a **virtual monitor** instead: GNOME creates an extra display for the cast and removes it again when the cast ends, so nothing on your own screens changes. Move windows onto it with `Super`+`Shift`+arrow; until you do, the television shows an empty desktop. This mode needs a Wayland session.
 2. A GStreamer pipeline captures the PipeWire stream, encodes H.264 video and AAC system audio, and writes a live HLS stream into `$XDG_RUNTIME_DIR/gnome-shell-cast/`.
 3. A tiny built-in HTTP server serves that stream on your LAN.
 4. The daemon tells the Chromecast (CASTv2 protocol) to play the stream URL.

TROUBLESHOOTING.md

@@ -245,6 +245,38 @@ The receiver app exited or never started rendering. Check the log for
 [The cast falls back to HLS](#the-cast-falls-back-to-hls-or-the-receiver-never-answers)
 below.
 
+### "New monitor" is missing from the menu
+
+That mode is only offered on a **Wayland** session, because GNOME on Xorg cannot
+create a monitor. Check which one you are on:
+
+```sh
+echo $XDG_SESSION_TYPE
+```
+
+If it says `x11`, log out and pick *GNOME* rather than *GNOME on Xorg* at the
+login screen. Casting the screen normally works either way.
+
+### The new monitor shows only the wallpaper
+
+Nothing has been moved onto it yet - a new monitor starts empty, the same as
+plugging in a second screen. Move the window you want with
+`Super`+`Shift`+`Right`, or drag it past the edge of your own screen. *Settings →
+Displays* shows where GNOME placed the new monitor relative to the others.
+
+### `The system could not create a new monitor for this cast.`
+
+GNOME accepted the request and then failed to make the monitor. Almost always an
+Xorg session (see above); otherwise the compositor refused the size. The real
+reason is in the shell's own log, not the daemon's:
+
+```sh
+journalctl --user -b -u org.gnome.shell.service | grep -i 'virtual monitor'
+```
+
+`Backend doesn't support creating virtual monitors` confirms Xorg. Cast the
+screen normally in the meantime.
+
 ---
 
 ## Connection