Cursor: keep the system pointer where no cursor theme loads on Wayland (#2450)

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

apps/vectorcraft/src/cursor_theme.rs

@@ -7,11 +7,15 @@
 //! 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.
 //!
+//! Where no cursor theme can be found at all (KDE or sway without gsettings), the env fix does
+//! nothing: [`theme_missing`] tells the canvas to keep the system pointer visible beside the tool glyph.
+//!
 //! 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;
+use std::path::{Path, PathBuf};
 
 /// 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"];
@@ -48,6 +52,67 @@ pub fn plan(var: impl Fn(&str) -> Option<OsString>, host_icons: &[&str], desktop
     out
 }
 
+/// Xcursor's default search path when `XCURSOR_PATH` is unset (relative entries are in `$HOME`).
+const DEFAULT_ICON_DIRS: [&str; 5] = ["~/.local/share/icons", "~/.icons", "/usr/share/icons", "/usr/share/pixmaps", "/usr/X11R6/lib/X11/icons"];
+
+/// The themes `index.theme` lists after `Inherits=`.
+fn inherits(index: &str) -> Vec<String> {
+    index
+        .lines()
+        .find_map(|l| l.trim().strip_prefix("Inherits="))
+        .map(|v| v.split([',', ';']).map(|t| t.trim().to_owned()).filter(|t| !t.is_empty()).collect())
+        .unwrap_or_default()
+}
+
+/// Can Xcursor load a pointer for the environment's theme (`XCURSOR_THEME`, else `default`)? It does
+/// when the theme, or one it inherits from, has a `default` or `left_ptr` cursor file in a folder of
+/// `XCURSOR_PATH` (else Xcursor's default folders). `read` returns a file's text, `exists` tells
+/// whether a file is there.
+pub fn theme_usable(
+    var: impl Fn(&str) -> Option<OsString>,
+    home: Option<&Path>,
+    read: impl Fn(&Path) -> Option<String>,
+    exists: impl Fn(&Path) -> bool,
+) -> bool {
+    let dirs: Vec<PathBuf> = match var("XCURSOR_PATH").filter(|v| !v.is_empty()) {
+        Some(v) => std::env::split_paths(&v).collect(),
+        None => DEFAULT_ICON_DIRS
+            .iter()
+            .filter_map(|d| match d.strip_prefix("~/") {
+                Some(rest) => home.map(|h| h.join(rest)),
+                None => Some(PathBuf::from(d)),
+            })
+            .collect(),
+    };
+    let first = var("XCURSOR_THEME").and_then(|v| v.into_string().ok()).filter(|t| !t.is_empty()).unwrap_or_else(|| "default".into());
+    let mut todo = vec![first];
+    let mut seen: Vec<String> = Vec::new();
+    while let Some(theme) = todo.pop() {
+        // Bounded: themes inherit in short chains, and a cycle must end.
+        if seen.contains(&theme) || seen.len() >= 16 {
+            continue;
+        }
+        for dir in &dirs {
+            let base = dir.join(&theme);
+            if ["default", "left_ptr"].iter().any(|c| exists(&base.join("cursors").join(c))) {
+                return true;
+            }
+            if let Some(index) = read(&base.join("index.theme")) {
+                todo.extend(inherits(&index));
+            }
+        }
+        seen.push(theme);
+    }
+    false
+}
+
+/// On this machine, after [`apply`]: is there no cursor theme Xcursor can load? Then winit never
+/// sets a pointer, and the canvas must not hide the system one (#2450).
+pub fn theme_missing() -> bool {
+    let home = std::env::var_os("HOME").filter(|h| !h.is_empty()).map(PathBuf::from);
+    !theme_usable(|n| std::env::var_os(n), home.as_deref(), |p| std::fs::read_to_string(p).ok(), |p| p.exists())
+}
+
 /// The value of a `gsettings get` line: `'Adwaita'` becomes `Adwaita`, `24` stays `24`.
 fn gsettings_value(output: &str) -> &str {
     output.trim().trim_matches('\'')
@@ -113,6 +178,27 @@ mod tests {
         assert!(plan(env(&[("WAYLAND_DISPLAY", "wayland-0")]), &[], odd).is_empty());
     }
 
+    #[test]
+    fn a_theme_is_usable_directly_or_through_inherits_and_missing_otherwise() {
+        let files = ["/i/Bibata/cursors/left_ptr", "/i/Breeze/index.theme", "/i/Loop/index.theme"];
+        let exists = |p: &Path| p.to_str().is_some_and(|p| files.contains(&p));
+        let read = |p: &Path| match p.to_str()? {
+            "/i/Breeze/index.theme" => Some("[Icon Theme]\nInherits=Other, Bibata\n".to_owned()),
+            "/i/Loop/index.theme" => Some("Inherits=Loop\n".to_owned()),
+            _ => None,
+        };
+        let with = |theme: &'static str| {
+            move |n: &str| (n == "XCURSOR_PATH").then(|| OsString::from("/i")).or((n == "XCURSOR_THEME").then(|| OsString::from(theme)))
+        };
+        assert!(theme_usable(with("Bibata"), None, read, exists));
+        assert!(theme_usable(with("Breeze"), None, read, exists));
+        // No such theme, a theme without cursors, an inheritance loop: nothing to load.
+        assert!(!theme_usable(with("Nope"), None, read, exists));
+        assert!(!theme_usable(with("Loop"), None, read, exists));
+        // Unset: Xcursor's default folders and the `default` theme.
+        assert!(!theme_usable(|_| None, Some(Path::new("/home/u")), read, exists));
+    }
+
     #[test]
     fn gsettings_output_is_unquoted() {
         assert_eq!(gsettings_value("'Adwaita'\n"), "Adwaita");

apps/vectorcraft/src/main.rs

@@ -526,6 +526,7 @@ fn services(parent: Parent, ui: egui::Context) -> Services {
         start_pick: cfg!(unix).then(|| Box::new(move |request: PickRequest| start_pick(request, &p5, &ui)) as vectorcraft_ui_egui::picks::StartPick),
         dialogs_are_sheets: cfg!(target_os = "macos"),
         wayland,
+        no_cursor_theme: wayland && cursor_theme::theme_missing(),
         read: Some(Box::new(|p: &str| std::fs::read(p).map_err(|e| e.to_string()))),
         write: Some(Box::new(write_file)),
         // Background Save and Export write from a worker thread.