Tabs: the ruler and Repeat Tab stop at 256 tab stops

With 256 stops, a click on an empty part of the Tabs panel's ruler adds
no stop and shows "at most 256 tab stops" in the status bar; the stops
and the selection stay as they were. Repeat Tab adds copies only up to
256 stops and shows the same message when it stops early. Before, both
built a longer list that text.tabs.set refused, so nothing changed and
no message showed.

crates/ui-egui/src/panels/tabs.rs

@@ -686,15 +686,22 @@ pub fn show(app: &mut VectorcraftApp, ui: &mut Ui) {
                     // A click that lands on a stop's whole point or tick selects that stop and records no
                     // undo step.
                     let added = !stops.iter().any(|s| s.position == position);
-                    if added {
-                        stops.push(TabStop { position, align: ALIGNS[align_idx.min(3)].0, leader: String::new(), align_on: '.' });
-                        stops.sort_by(|a, b| a.position.total_cmp(&b.position));
-                    }
-                    let i = stops.iter().position(|s| s.position == position).unwrap_or(0);
-                    set_pstate::<Option<IndentMarker>>(ui.ctx(), "tabs-marker-sel", None);
-                    set_pstate(ui.ctx(), "tabs-sel", i);
-                    if added {
-                        apply(app, &stops);
+                    let max = TabStop::MAX_STOPS;
+                    if added && stops.len() >= max {
+                        // A full list takes no more stops: the click leaves the stops and the
+                        // selection as they are and shows the limit.
+                        app.status(format!("at most {max} tab stops"));
+                    } else {
+                        if added {
+                            stops.push(TabStop { position, align: ALIGNS[align_idx.min(3)].0, leader: String::new(), align_on: '.' });
+                            stops.sort_by(|a, b| a.position.total_cmp(&b.position));
+                        }
+                        let i = stops.iter().position(|s| s.position == position).unwrap_or(0);
+                        set_pstate::<Option<IndentMarker>>(ui.ctx(), "tabs-marker-sel", None);
+                        set_pstate(ui.ctx(), "tabs-sel", i);
+                        if added {
+                            apply(app, &stops);
+                        }
                     }
                 }
             }
@@ -729,7 +736,7 @@ pub fn menu(app: &mut VectorcraftApp, ui: &mut Ui) {
     }
     // Repeat Tab: copies of the selected stop across the ruler, spaced by its distance from the
     // previous stop, or from the paragraph's left indent plus its list level's indent when it is
-    // the first stop.
+    // the first stop. The copies stop at the 256-stop limit, which the status bar then shows.
     if menu_item(ui, tl!("Repeat Tab"), sel < stops.len() && selected_marker.is_none(), false) {
         let step = match sel {
             0 => (stops[0].position - left_indent).abs(),
@@ -738,8 +745,13 @@ pub fn menu(app: &mut VectorcraftApp, ui: &mut Ui) {
         .max(1.0);
         let base = stops[sel].clone();
         stops.truncate(sel + 1);
+        let max = TabStop::MAX_STOPS;
         let mut pos = base.position + step;
         while pos <= base.position + step * 20.0 && pos < 2000.0 {
+            if stops.len() >= max {
+                app.status(format!("at most {max} tab stops"));
+                break;
+            }
             stops.push(TabStop { position: pos, ..base.clone() });
             pos += step;
         }

crates/ui-egui/src/panels/tabs/tests.rs

@@ -1255,6 +1255,35 @@ fn panel_menu_does_not_change_stops_while_an_indent_marker_is_selected() {
     assert_eq!(current(&app).unwrap().1, before);
 }
 
+/// With 256 stops, a click on an empty part of the ruler adds none and shows the limit in the
+/// status bar. Repeat Tab near the end of the list adds copies only up to 256 stops and shows the
+/// limit too.
+#[test]
+fn the_ruler_and_repeat_tab_stop_at_256_stops() {
+    let mut app = VectorcraftApp::new(Session::new(), Default::default());
+    app.session.execute("file.new", &json!({})).unwrap();
+    app.session.execute("text.create", &json!({"x": 10, "y": 20, "text": "a	b", "area": {"width": 300, "height": 100}})).unwrap();
+    // 250 stops 1 pt apart from 0 pt, then six 2 pt apart from 260 pt.
+    let start: Vec<f64> = (0..250).map(f64::from).chain((0..6).map(|i| 260.0 + 2.0 * f64::from(i))).collect();
+    let stops: Vec<Value> = start.iter().map(|p| json!({"position": p})).collect();
+    app.session.execute("text.tabs.set", &json!({ "stops": stops })).unwrap();
+    let ctx = egui::Context::default();
+    let positions = |app: &VectorcraftApp| current(app).unwrap().1.iter().map(|s| s.position).collect::<Vec<_>>();
+    set_pstate(&ctx, "tabs-sel", 3usize);
+    drag_on_ruler(&ctx, &mut app, 290.0, 290.0, false);
+    assert_eq!(positions(&app), start);
+    assert_eq!(pstate::<usize>(&ctx, "tabs-sel"), 3);
+    assert_eq!(app.ui.status, "at most 256 tab stops");
+    // Repeat Tab on the stop at 260 pt (11 pt past the one before it): five copies, not 20.
+    app.status("");
+    set_pstate(&ctx, "tabs-sel", 250usize);
+    choose(&ctx, &mut app, "Repeat Tab");
+    let repeated = positions(&app);
+    assert_eq!(repeated.len(), 256);
+    assert_eq!(repeated[249..], [249.0, 260.0, 271.0, 282.0, 293.0, 304.0, 315.0]);
+    assert_eq!(app.ui.status, "at most 256 tab stops");
+}
+
 /// Repeat Tab on the first stop spaces the copies by the stop's distance from the paragraph's
 /// left indent (18 pt further in for each list level past the first), from the paragraph the
 /// caret is in while the Type tool edits text.