diff --git a/crates/engine/src/library.rs b/crates/engine/src/library.rs
index c98ef4617..6233737f2 100644
--- a/crates/engine/src/library.rs
+++ b/crates/engine/src/library.rs
@@ -272,6 +272,32 @@ impl SettingsLoad {
 }
 
 impl Session {
+    /// Check, without closing the library open now, that [`Session::open_library`] can be expected
+    /// to open `dir`: not a Lightroom catalog, not some other file, a readable folder (or one not
+    /// made yet), not locked by another program. Returns the library folder ([`resolve_dir`]).
+    /// Nothing is created; a library folder that exists gets its lock taken and released again.
+    /// Opening can still fail afterwards (a damaged catalog, a disk going away).
+    pub fn check_library(&self, dir: impl AsRef<Path>) -> Result<PathBuf> {
+        let dir = dir.as_ref().to_path_buf();
+        if let Some(place) = crate::lightroom_folder::lightroom_catalog_at(&dir) {
+            return Err(EngineError::LightroomCatalog { path: dir, catalog: place.catalog });
+        }
+        let dir = resolve_dir(&dir)?;
+        if !dir.exists() {
+            return Ok(dir);
+        }
+        if !dir.is_dir() {
+            return Err(EngineError::Other(format!("{} is not a folder", dir.display())));
+        }
+        std::fs::read_dir(&dir).map_err(|e| EngineError::Other(format!("can't open library {}: {e}", dir.display())))?;
+        // the library open in this session keeps its lock when reopened; any other must be free
+        let current = self.library.as_ref().is_some_and(|l| l.on_disk && same_dir(&l.dir, &dir));
+        if !current && is_library(&dir) {
+            drop(LibraryLock::acquire(&dir, &program_name()).map_err(|e| EngineError::LibraryInUse(e.to_string()))?);
+        }
+        Ok(dir)
+    }
+
     /// Open (or create) the library at `dir` into this session, replacing its catalog. With
     /// `seed_demo`, a newly created library starts with the procedural demo photos.
     ///
@@ -721,4 +747,32 @@ mod resolve_tests {
         assert!(!e.contains("File exists"), "{e}");
         let _ = std::fs::remove_dir_all(&dir);
     }
+
+    /// `check_library` refuses what `open_library` would, without touching the open library.
+    #[test]
+    fn check_library_refuses_locked_libraries_files_and_lightroom_catalogs() {
+        let dir = std::env::temp_dir().join(format!("lc-check-lib-{}", std::process::id()));
+        let _ = std::fs::remove_dir_all(&dir);
+        let (a, b) = (dir.join("a"), dir.join("b"));
+        std::fs::create_dir_all(&b).unwrap();
+        let mut s = Session::new().with_fs();
+        s.open_library(&b, false).unwrap();
+        s.close_library().unwrap();
+        s.open_library(&a, false).unwrap();
+        assert_eq!(s.check_library(&a).unwrap(), a, "the open library keeps its lock");
+        assert_eq!(s.check_library(&b).unwrap(), b);
+        assert_eq!(s.check_library(dir.join("new")).unwrap(), dir.join("new"));
+        assert!(!dir.join("new").exists(), "nothing created");
+        let held = LibraryLock::acquire(&b, "other").unwrap();
+        assert!(matches!(s.check_library(&b), Err(EngineError::LibraryInUse(_))));
+        drop(held);
+        assert!(s.check_library(&b).is_ok(), "the probe released its lock");
+        std::fs::write(dir.join("notes.txt"), b"x").unwrap();
+        assert!(s.check_library(dir.join("notes.txt")).is_err());
+        std::fs::write(dir.join("Cat.lrcat"), b"x").unwrap();
+        assert!(matches!(s.check_library(dir.join("Cat.lrcat")), Err(EngineError::LightroomCatalog { .. })));
+        assert!(s.library.as_ref().is_some_and(|l| l.dir == a));
+        drop(s);
+        let _ = std::fs::remove_dir_all(&dir);
+    }
 }
diff --git a/crates/ui-egui/src/panels/settings.rs b/crates/ui-egui/src/panels/settings.rs
index a668439c2..fefe8ef55 100644
--- a/crates/ui-egui/src/panels/settings.rs
+++ b/crates/ui-egui/src/panels/settings.rs
@@ -673,17 +673,27 @@ pub fn open_library(app: &mut LightcraftApp, p: &Value) -> Result<Value, String>
 }
 
 /// Close the current library and open the one at `path`. If it can't be opened (locked by
-/// another program, unreadable, a file), the error is returned and the current library stays open.
+/// another program, unreadable, a file), the error is returned and the current library stays open:
+/// the target is checked before anything is closed, and if opening it still fails the previous
+/// library is opened again (both errors are reported if that fails too).
 fn switch_library(app: &mut LightcraftApp, path: &str) -> Result<Value, String> {
-    let dir = lightcraft_engine::library::resolve_dir(std::path::Path::new(path)).map_err(|e| e.to_string())?;
+    let dir = app.session.check_library(std::path::Path::new(path)).map_err(|e| format!("{path}: {e}"))?;
     let path = dir.to_string_lossy().into_owned();
     // the library being left goes in the list too (also when it was opened at launch)
-    let previous = app.session.library.as_ref().filter(|l| l.on_disk).map(|l| l.dir.to_string_lossy().into_owned());
+    let previous_dir = app.session.library.as_ref().filter(|l| l.on_disk).map(|l| l.dir.clone());
+    let previous = previous_dir.as_ref().map(|d| d.to_string_lossy().into_owned());
     app.session.close_library().map_err(|e| e.to_string())?;
-    app.session.open_library(&dir, false).map_err(|e| format!("{path}: {e}"))?;
+    let opened = app.session.open_library(&dir, false).map(|_| ()).map_err(|e| format!("{path}: {e}"));
     app.lightroom_last = None;
     app.renderer.forget_all();
     app.ui.compare = None;
+    if let Err(err) = opened {
+        let Some(prev) = previous_dir else { return Err(err) };
+        return match app.session.open_library(&prev, false) {
+            Ok(_) => Err(err),
+            Err(e) => Err(format!("{err}; reopening the previous library {} failed too: {e}", prev.display())),
+        };
+    }
     app.ui.settings.library_path = path.clone();
     if let Some(prev) = previous.filter(|p| p != &path && !app.ui.settings.recent_libraries.contains(p)) {
         remember_library(&mut app.ui.settings.recent_libraries, &prev);
@@ -768,10 +778,19 @@ pub fn new_library(app: &mut LightcraftApp, p: &Value) -> Result<Value, String>
     switch_library(app, &dir.to_string_lossy())
 }
 
+/// Characters a folder name can't hold on Windows (and `/`, which no system allows).
+const RESERVED_NAME_CHARS: [char; 9] = ['<', '>', ':', '"', '/', '\\', '|', '?', '*'];
+
 fn valid_library_name(s: &str) -> Result<&str, String> {
     let n = s.trim();
-    if n.is_empty() || n == "." || n == ".." || n.contains(['/', '\\']) || n.chars().any(char::is_control) {
-        return Err(format!("app.newLibrary: {s:?} can't be a library name (a folder name, without / or \\)"));
+    // Windows drops trailing dots and spaces from a folder name: refuse them rather than
+    // creating a folder whose name differs from the one asked for (spaces around the name
+    // are trimmed first)
+    let bad = n.is_empty() || n == "." || n == ".." || n.ends_with(['.', ' ']) || n.contains(RESERVED_NAME_CHARS) || n.chars().any(char::is_control);
+    if bad {
+        return Err(format!(
+            "app.newLibrary: {s:?} can't be a library name (a folder name without < > : \" / \\ | ? * and not ending in a dot or space)"
+        ));
     }
     Ok(n)
 }
diff --git a/crates/ui-egui/src/tests_switch_library.rs b/crates/ui-egui/src/tests_switch_library.rs
index d45b4ea14..8c5208e3d 100644
--- a/crates/ui-egui/src/tests_switch_library.rs
+++ b/crates/ui-egui/src/tests_switch_library.rs
@@ -162,3 +162,41 @@ fn missing_recent_libraries_are_greyed_out_and_pruned_and_the_list_is_capped() {
     assert!(app.ui.settings.recent_libraries.is_empty());
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+/// A library another program has open is refused before the current one is closed: the current
+/// library stays open with its photos, and the window title keeps its name.
+#[test]
+fn switching_to_a_locked_library_keeps_the_current_one_open() {
+    let dir = temp("lockedlib");
+    let mut app = app_in(&dir);
+    app.run("app.newLibrary", json!({"path": dir.to_string_lossy(), "name": "Other"})).unwrap();
+    app.run("app.openLibrary", json!({"path": dir.join("first").to_string_lossy()})).unwrap();
+    // "another program" holds Other's lock
+    let other = dir.join("Other");
+    let held = lightcraft_catalog::LibraryLock::acquire(&other, "someone else").unwrap();
+    let e = app.run("app.openLibrary", json!({"path": other.to_string_lossy()})).unwrap_err();
+    assert!(e.contains("already open"), "{e}");
+    assert!(app.session.library.as_ref().unwrap().dir.ends_with("first"), "the current library stays");
+    assert!(app.session.library.as_ref().unwrap().on_disk);
+    assert!(app.ui.settings.library_path.ends_with("first"));
+    drop(held);
+    app.run("app.openLibrary", json!({"path": other.to_string_lossy()})).unwrap();
+    assert!(app.session.library.as_ref().unwrap().dir.ends_with("Other"));
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
+/// Library names that can't be folder names everywhere (Windows' reserved characters, a
+/// trailing dot) are refused; spaces around a name are trimmed.
+#[test]
+fn library_names_must_work_as_folder_names_on_every_system() {
+    let dir = temp("libnames");
+    let mut app = app_in(&dir);
+    for bad in ["a<b", "a>b", "a:b", "a\"b", "a/b", "a\\b", "a|b", "a?b", "a*b", "name.", "..", " ", "a\u{7}"] {
+        let e = app.run("app.newLibrary", json!({"path": dir.to_string_lossy(), "name": bad})).unwrap_err();
+        assert!(e.contains("library name"), "{bad:?}: {e}");
+    }
+    assert!(app.session.library.as_ref().unwrap().dir.ends_with("first"));
+    app.run("app.newLibrary", json!({"path": dir.to_string_lossy(), "name": "  Trip 2026  "})).unwrap();
+    assert_eq!(app.session.library.as_ref().unwrap().dir, dir.join("Trip 2026"));
+    let _ = std::fs::remove_dir_all(&dir);
+}