Show: Presenter View only with a second display (#1156)

* Show: Presenter View only with a second display

With one display the presenter window opened on the same screen as the show. Under Hyprland's
focus-follows-mouse, whichever window the pointer was over took full screen, and the hidden one
stalled every frame: the show lagged, dropped key presses or froze.

One display now shows only the slides, as in PowerPoint. The host counts displays through winit's
monitor list when a show starts; hosts that can't tell keep Presenter View.

* Show: count displays by MonitorHandle

Wayland lists one output twice under the same wl_output id, so comparing handles counts it once, and two separate displays with the same name and geometry still count as two.

* Show: docs for Presenter View with a second display

slideshow-parity and hardware-parity note that on Linux one display shows only the slides, with status lines and revision history; ROADMAP progress log. display_count collects into a BTreeSet (MonitorHandle's Ord matches its Eq).

* Show: docs state the one-display rule exactly

Presenter View stays off when winit lists exactly one display; if it lists none, the setting is kept. The host callback covers Unix other than macOS, so the docs say Linux and the BSDs.

* Apply review fixes

- docs/slideshow-parity.md

---------

Co-authored-by: aiongg <67543387+aiongg@users.noreply.github.com>

apps/deckcraft/src/main.rs

@@ -143,6 +143,14 @@ fn services(native_wayland: bool) -> Services {
     }
 }
 
+/// Displays winit lists for `window`, counting an output Wayland lists twice once. `None` when it
+/// lists none: Presenter View then stays as it was.
+#[cfg(all(unix, not(target_os = "macos")))]
+fn display_count(window: &winit::window::Window) -> Option<usize> {
+    let n = window.available_monitors().collect::<std::collections::BTreeSet<_>>().len();
+    (n > 0).then_some(n)
+}
+
 const APP_ID: &str = "ai.storyteller.deckcraft";
 
 const USAGE: &str = "\
@@ -280,6 +288,11 @@ fn main() -> eframe::Result {
         Box::new(move |cc| {
             deckcraft_ui_egui::install_ime_pass_hooks(&cc.egui_ctx);
             let mut app = SlideApp::new(Session::new(), services(native_wayland));
+            // Presenter View asks winit how many displays there are when a show starts.
+            #[cfg(all(unix, not(target_os = "macos")))]
+            if let Some(window) = cc.winit_window().cloned() {
+                app.services.displays = Some(Box::new(move || display_count(&window)));
+            }
             load_prefs(&mut app);
             app.enable_update_checks();
             // AutoRecover: reopen what a previous run left unsaved, then keep it current.