Keep the File > New tiles inside the right margin in a narrow window (#1421)

set_max_width(CONTENT_W) widened the page past the margin whenever the window was
narrower than CONTENT_W, so the last tile in each row ran off the screen edge.

crates/ui-egui/src/backstage.rs

@@ -389,7 +389,9 @@ pub fn show(app: &mut SlideApp, ui: &mut Ui) {
         w.active.bg_stroke = Stroke::new(1.0, t.accent);
         egui::ScrollArea::vertical().auto_shrink([false, false]).show(ui, |ui| {
             egui::Frame::NONE.inner_margin(egui::Margin { left: 48, right: 36, top: 22, bottom: 32 }).show(ui, |ui| {
-                ui.set_max_width(CONTENT_W);
+                // set_max_width can also widen: in a window narrower than CONTENT_W it would push the
+                // page past the right margin and clip the tiles (#1421).
+                ui.set_max_width(ui.available_width().min(CONTENT_W));
                 if bs.page != Page::Home {
                     ui.label(RichText::new(bs.page.label()).font(theme::bold(22.0)).color(t.text));
                     ui.add_space(16.0);

crates/ui-egui/src/backstage/tests.rs

@@ -179,6 +179,25 @@ fn every_page_renders() {
     assert!(h.state_mut().run("app.backstage", json!({"page": "share"})).is_err(), "Share (cloud) isn't offered");
 }
 
+/// In a window narrower than the page's maximum width the New tiles stay inside the right margin (#1421).
+#[test]
+fn new_tiles_keep_the_right_margin_in_a_narrow_window() {
+    let mut h = Harness::builder().with_size(egui::vec2(1000.0, 900.0)).build_ui_state(
+        |ui, app: &mut SlideApp| {
+            app.logic(ui.ctx());
+            app.ui(ui);
+        },
+        new_app(),
+    );
+    h.state_mut().run("app.backstage", json!({"page": "new"})).unwrap();
+    h.run_steps(3);
+    let tiles = ["DeckCraft", "Harbor", "Ember", "Meadow", "Nocturne", "Paper", "Briefing"];
+    for label in tiles {
+        let r = h.get_by_label(label).rect();
+        assert!(r.right() <= 1000.0 - 36.0, "{label} ends at {}, inside the 36pt right margin", r.right());
+    }
+}
+
 /// A starter template from File › New opens its deck and leaves the File tab.
 #[test]
 fn new_template_opens_a_starter_deck() {