Wayland: give winit the desktop cursor theme so the pointer returns over panels (#2450)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/vectorcraft/src/cursor_theme.rs

@@ -0,0 +1,121 @@
+//! Linux Wayland: tell winit where the cursor theme is (#2450).
+//!
+//! Without the cursor-shape protocol (GNOME 46 and others) winit loads the pointer from an Xcursor
+//! theme itself. When that lookup finds nothing, no pointer is ever set and the compositor keeps
+//! showing its default one, until the canvas hides the system pointer to paint a tool glyph
+//! (`CursorIcon::None`). Then the arrow never comes back over the panels and menus. The lookup
+//! fails in a Flatpak sandbox (`XCURSOR_PATH` unset, no cursor theme in the runtime) and where the
+//! `default` theme is a dangling symlink; the desktop's own setting names a theme that loads.
+//!
+//! This reads the desktop's cursor theme and size and sets what the user left unset, once, before
+//! any thread starts and before the window opens.
+#![cfg_attr(not(target_os = "linux"), allow(dead_code))]
+
+use std::ffi::OsString;
+
+/// Where a Flatpak sandbox mounts the host's cursor themes (user and system).
+const FLATPAK_HOST_ICONS: [&str; 2] = ["/run/host/user-share/icons", "/run/host/share/icons"];
+
+/// The desktop's cursor settings (GNOME's `org.gnome.desktop.interface`).
+#[derive(Debug, Default, PartialEq)]
+pub struct DesktopCursor {
+    pub theme: Option<String>,
+    pub size: Option<u32>,
+}
+
+/// The variables to set, given the environment, the host icon folders that exist and the
+/// desktop's cursor settings. Empty unless this is a Wayland session; variables the user set
+/// stay as they are.
+pub fn plan(var: impl Fn(&str) -> Option<OsString>, host_icons: &[&str], desktop: DesktopCursor) -> Vec<(&'static str, OsString)> {
+    let set = |name: &str| var(name).is_some_and(|v| !v.is_empty());
+    if !(set("WAYLAND_DISPLAY") || set("WAYLAND_SOCKET")) {
+        return Vec::new();
+    }
+    let mut out = Vec::new();
+    if !set("XCURSOR_PATH") && !host_icons.is_empty() {
+        out.push(("XCURSOR_PATH", OsString::from(host_icons.join(":"))));
+    }
+    if !set("XCURSOR_THEME")
+        && let Some(theme) = desktop.theme.filter(|t| !t.is_empty())
+    {
+        out.push(("XCURSOR_THEME", OsString::from(theme)));
+    }
+    if !set("XCURSOR_SIZE")
+        && let Some(size) = desktop.size.filter(|s| (1..=512).contains(s))
+    {
+        out.push(("XCURSOR_SIZE", OsString::from(size.to_string())));
+    }
+    out
+}
+
+/// The value of a `gsettings get` line: `'Adwaita'` becomes `Adwaita`, `24` stays `24`.
+fn gsettings_value(output: &str) -> &str {
+    output.trim().trim_matches('\'')
+}
+
+#[cfg(target_os = "linux")]
+fn desktop_cursor() -> DesktopCursor {
+    let get = |key: &str| {
+        let out = std::process::Command::new("gsettings").args(["get", "org.gnome.desktop.interface", key]).output().ok()?;
+        out.status.success().then(|| String::from_utf8_lossy(&out.stdout).into_owned())
+    };
+    DesktopCursor {
+        theme: get("cursor-theme").map(|v| gsettings_value(&v).to_owned()),
+        size: get("cursor-size").and_then(|v| gsettings_value(&v).parse().ok()),
+    }
+}
+
+/// Sets `XCURSOR_PATH`, `XCURSOR_THEME` and `XCURSOR_SIZE` where a Wayland session leaves them
+/// unset. Call first thing in `main`, while the process is single-threaded.
+#[cfg(target_os = "linux")]
+#[allow(unsafe_code)]
+pub fn apply() {
+    let host: Vec<&str> = FLATPAK_HOST_ICONS.iter().copied().filter(|p| std::path::Path::new(p).is_dir()).collect();
+    for (name, value) in plan(|n| std::env::var_os(n), &host, desktop_cursor()) {
+        log::debug!("cursor theme: {name}={value:?}");
+        // SAFETY: called at the top of `main`, before the app starts any thread.
+        unsafe { std::env::set_var(name, value) };
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn env<'a>(pairs: &'a [(&'a str, &'a str)]) -> impl Fn(&str) -> Option<OsString> + 'a {
+        move |name| pairs.iter().find(|(k, _)| *k == name).map(|(_, v)| OsString::from(v))
+    }
+
+    fn adwaita() -> DesktopCursor {
+        DesktopCursor { theme: Some("Adwaita".into()), size: Some(24) }
+    }
+
+    #[test]
+    fn a_wayland_session_gets_the_desktop_theme_and_the_host_icons() {
+        let got = plan(env(&[("WAYLAND_DISPLAY", "wayland-0")]), &["/run/host/share/icons"], adwaita());
+        assert_eq!(
+            got,
+            vec![
+                ("XCURSOR_PATH", OsString::from("/run/host/share/icons")),
+                ("XCURSOR_THEME", OsString::from("Adwaita")),
+                ("XCURSOR_SIZE", OsString::from("24")),
+            ]
+        );
+    }
+
+    #[test]
+    fn settings_the_user_made_and_other_sessions_stay_untouched() {
+        let set = [("WAYLAND_DISPLAY", "wayland-0"), ("XCURSOR_PATH", "/x"), ("XCURSOR_THEME", "Bibata"), ("XCURSOR_SIZE", "32")];
+        assert!(plan(env(&set), &["/run/host/share/icons"], adwaita()).is_empty());
+        assert!(plan(env(&[("DISPLAY", ":0")]), &["/run/host/share/icons"], adwaita()).is_empty());
+        // A theme-less desktop and an absurd size set nothing.
+        let odd = DesktopCursor { theme: Some(String::new()), size: Some(0) };
+        assert!(plan(env(&[("WAYLAND_DISPLAY", "wayland-0")]), &[], odd).is_empty());
+    }
+
+    #[test]
+    fn gsettings_output_is_unquoted() {
+        assert_eq!(gsettings_value("'Adwaita'\n"), "Adwaita");
+        assert_eq!(gsettings_value("24\n"), "24");
+    }
+}

apps/vectorcraft/src/main.rs

@@ -27,6 +27,7 @@ mod clipboard;
 #[cfg(feature = "wgpu")]
 mod compositor_gpu;
 mod control_server;
+mod cursor_theme;
 #[cfg(feature = "wgpu")]
 mod gpu;
 mod logging;
@@ -654,6 +655,9 @@ Without a window (converting, scripting, the MCP server), use vectorcraft-cli:
 ";
 
 fn main() -> std::process::ExitCode {
+    // Before any thread starts: the pointer must find a cursor theme on Wayland (#2450).
+    #[cfg(target_os = "linux")]
+    cursor_theme::apply();
     // Validate roots before logging, font scans, preferences or the window: a missing value
     // must not clear an environment root and start the control channel without confinement.
     let mut control = automation_args::ControlArg::default();

packaging/linux/flatpak/ai.storyteller.vectorcraft.bundle.yml

@@ -24,6 +24,10 @@ finish-args:
   - --socket=fallback-x11
   # GPU access for the wgpu renderer (Vulkan, or GL as a fallback).
   - --device=dri
+  # Cursor theme: without the cursor-shape protocol (e.g. GNOME 46) winit loads the theme itself.
+  # The sandbox sets no XCURSOR_PATH and ships no cursor themes, so the pointer would turn invisible
+  # over the window. The host's themes are mounted under /run/host (#2450).
+  - --env=XCURSOR_PATH=/run/host/user-share/icons:/run/host/share/icons
   # Open/Save dialogs use the file-chooser portal (rfd), and drag-and-drop uses the document
   # portal, so broad filesystem access isn't needed. Pictures is granted for opening files by
   # path from the command line.

packaging/linux/flatpak/ai.storyteller.vectorcraft.yml

@@ -22,6 +22,10 @@ finish-args:
   - --socket=wayland
   - --socket=fallback-x11
   - --device=dri
+  # Cursor theme: without the cursor-shape protocol (e.g. GNOME 46) winit loads the theme itself.
+  # The sandbox sets no XCURSOR_PATH and ships no cursor themes, so the pointer would turn invisible
+  # over the window. The host's themes are mounted under /run/host (#2450).
+  - --env=XCURSOR_PATH=/run/host/user-share/icons:/run/host/share/icons
   # Open/Save dialogs use the file-chooser portal (rfd), and drag-and-drop uses the document
   # portal, so broad filesystem access isn't needed. Pictures is granted for opening files by
   # path from the command line.