diff --git a/crates/ui-egui/src/control.rs b/crates/ui-egui/src/control.rs
index 14d6fe831..68bba9ade 100644
--- a/crates/ui-egui/src/control.rs
+++ b/crates/ui-egui/src/control.rs
@@ -327,7 +327,7 @@ pub fn handle(app: &mut VectorcraftApp, ctx: &egui::Context, req: &ControlReques
                     return err(format!("the `{}` dialog has no field `{f}`; its fields: {names}", d.kind));
                 }
                 for (f, value) in fields {
-                    d.fields.insert(f, value);
+                    crate::dialogs::set_field(d, f, value);
                 }
                 ok(serde_json::to_value(&*d).unwrap_or_default())
             }
diff --git a/crates/ui-egui/src/dialogs.rs b/crates/ui-egui/src/dialogs.rs
index 49dfe37b9..6920c8466 100644
--- a/crates/ui-egui/src/dialogs.rs
+++ b/crates/ui-egui/src/dialogs.rs
@@ -410,6 +410,15 @@ pub fn accepts_field(d: &Dialog, field: &str) -> bool {
         || (spec.passes)(d, field)
 }
 
+/// Set `field` of `d` to `value` for `ui.dialog.set`, as the dialog's own control for it does: a
+/// Pattern Brush Options corner tile set this way is saved by OK, as one chosen from its menu is.
+pub fn set_field(d: &mut Dialog, field: String, value: Value) {
+    if d.kind == brush_options::KIND {
+        brush_art_options::note_set(d, &field);
+    }
+    d.fields.insert(field, value);
+}
+
 /// The fields `ui.dialog.set` can set on `d`, sorted, without the `__` ones: those it holds,
 /// `discard`, `preview` and the spec's `inputs` for its kind.
 pub fn field_names(d: &Dialog) -> Vec<String> {
diff --git a/crates/ui-egui/src/dialogs/brush_art_options.rs b/crates/ui-egui/src/dialogs/brush_art_options.rs
index 2c150b4bd..f9b5bf558 100644
--- a/crates/ui-egui/src/dialogs/brush_art_options.rs
+++ b/crates/ui-egui/src/dialogs/brush_art_options.rs
@@ -63,6 +63,24 @@ fn corner_art_field<'a>(def: Option<&Value>, field: &'a str) -> Option<&'a str>
     }
 }
 
+/// The dialog field that records that corner tile `key`'s menu was set.
+fn corner_set_key(key: &str) -> String {
+    format!("__{key}Set")
+}
+
+/// Sets corner tile `key` to `choice` from its menu.
+pub(super) fn choose_corner(d: &mut Dialog, key: &str, choice: &str) {
+    d.fields.insert(key.into(), json!(choice));
+    note_set(d, key);
+}
+
+/// Records that field `field` was set, from its menu or `ui.dialog.set`, when it is a corner tile.
+pub(super) fn note_set(d: &mut Dialog, field: &str) {
+    if CORNER_TILES.iter().any(|(key, ..)| *key == field) {
+        d.fields.insert(corner_set_key(field), json!(true));
+    }
+}
+
 /// The most key colours offered.
 const MAX_COLORS: usize = 16;
 
@@ -236,14 +254,21 @@ pub(super) fn params(d: &Dialog) -> Value {
             });
             // None or an automatic tile replaces the corner art. Original keeps the art the corner
             // draws, the other kind of corner's art included, so changing the other kind leaves it.
+            // A None the menu was not set to, on a corner without art or a source, leaves the
+            // definition as it is: that corner still draws the other kind's art when it gets some.
             let def = d.fields.get("__def");
-            for (key, art, source, _) in CORNER_TILES {
+            for (key, art, source, other) in CORNER_TILES {
                 let choice = d.str(key);
+                let unset = |f: &str| def.and_then(|d| d.get(f)).is_none_or(Value::is_null);
+                if choice == CornerSource::None.id() && !d.bool(&corner_set_key(key)) && unset(source) && corner_art_field(def, art).is_none() {
+                    continue;
+                }
                 if let Some(s) = CornerSource::parse(&choice) {
                     p[art] = Value::Null;
                     p[source] = json!(s.id());
                 } else if choice == "original"
                     && let Some(from) = corner_art_field(def, art).filter(|f| *f != art)
+                    && CORNER_TILES.iter().find(|(_, a, _, _)| *a == other).is_some_and(|(k, ..)| d.str(k) != "original")
                 {
                     p[art] = def.and_then(|d| d.get(from)).cloned().unwrap_or_default();
                 }
@@ -480,7 +505,7 @@ fn tiles(ui: &mut egui::Ui, d: &mut Dialog) {
                     if let Some((id, _)) =
                         widgets::dropdown_named(ui, ("brush-pattern-corner", key), Some(name), current, &labels, 76.0).and_then(|i| choices.get(i))
                     {
-                        d.fields.insert(key.into(), json!(id));
+                        choose_corner(d, key, id);
                     }
                 }
             });
diff --git a/crates/ui-egui/src/dialogs/brush_options/tests.rs b/crates/ui-egui/src/dialogs/brush_options/tests.rs
index 094a49f1e..9adeffed1 100644
--- a/crates/ui-egui/src/dialogs/brush_options/tests.rs
+++ b/crates/ui-egui/src/dialogs/brush_options/tests.rs
@@ -509,6 +509,44 @@ fn pattern_options_keep_the_corner_art_a_corner_borrows() {
     assert_eq!(def["outer_source"], json!("none"));
 }
 
+/// OK leaves the corner fields of a corner tile the menus were not set to as they were: a corner
+/// without art keeps borrowing the other kind's art when it gets some, and a corner that borrows
+/// art still borrows it. A None picked from the menu is saved, after another choice too.
+#[test]
+fn pattern_options_ok_leaves_corner_tiles_the_menus_were_not_set_to() {
+    let mut app = app();
+    app.run("ui.brushOptions", json!({"name": "Diamonds"})).unwrap();
+    set(&mut app, json!({"scale": 150}));
+    super::super::confirm(&mut app).unwrap();
+    let def = app.run("brush.get", json!({"name": "Diamonds"})).unwrap();
+    assert!(def.get("outer_source").is_none() && def.get("inner_source").is_none(), "{def}");
+    let art = app.run("brush.get", json!({"name": "Chain"})).unwrap()["outer_corner"].clone();
+    app.run("brush.options", json!({"name": "Diamonds", "params": {"outer_corner": art.clone()}})).unwrap();
+    app.run("ui.brushOptions", json!({"name": "Diamonds"})).unwrap();
+    assert_eq!(app.ui.dialog.as_ref().unwrap().str("innerCornerTile"), "original", "the inner corners draw the outer art");
+    super::super::confirm(&mut app).unwrap();
+    let def = app.run("brush.get", json!({"name": "Diamonds"})).unwrap();
+    assert!(def["outer_corner"].is_object() && def.get("inner_corner").is_none(), "{def}");
+
+    app.run("brush.new", json!({"type": "pattern", "name": "Plain", "params": {"side": art}})).unwrap();
+    app.run("ui.brushOptions", json!({"name": "Plain"})).unwrap();
+    let d = app.ui.dialog.as_mut().unwrap();
+    crate::dialogs::brush_art_options::choose_corner(d, "outerCornerTile", "autoCentered");
+    crate::dialogs::brush_art_options::choose_corner(d, "outerCornerTile", "none");
+    super::super::confirm(&mut app).unwrap();
+    let def = app.run("brush.get", json!({"name": "Plain"})).unwrap();
+    assert_eq!(def["outer_source"], json!("none"));
+    assert!(def.get("inner_source").is_none(), "{def}");
+    // None set through the control channel is saved too.
+    app.run("ui.brushOptions", json!({"name": "Plain"})).unwrap();
+    let (set, _) = crate::control::ControlRequest::new("ui.dialog.set", json!({"field": "innerCornerTile", "value": "none"}));
+    let crate::control::Outcome::Done(r) = crate::control::handle(&mut app, &egui::Context::default(), &set) else { panic!("not done") };
+    assert!(r.get("error").is_none(), "{r}");
+    super::super::confirm(&mut app).unwrap();
+    let def = app.run("brush.get", json!({"name": "Plain"})).unwrap();
+    assert_eq!(def["inner_source"], json!("none"));
+}
+
 /// Art Brush Options' Width and Pattern Brush Options' Scale each have a slider beside their
 /// field, which screen readers name as the row, with the field's number and text over the
 /// value's whole range.