Fix #563: the font menu's live preview no longer flickers

Hovering a family in a font menu previews it on the selected text. The
menu took its caller's font as the current family, and the caller reads
it from the text, which the preview had just changed: the next frame the
hovered family looked like the current one, so the menu ended the
preview, and the frame after it previewed the family again. The text
switched fonts every frame (and the document's revision ran up with it).

The menu now keeps the family the text had as it opened as the current
one: a preview stays on the text until another row is hovered, the
current family is hovered again, or the menu closes.

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

crates/ui-egui/src/font_menu.rs

@@ -134,6 +134,10 @@ struct MenuState {
     /// The pass the list was last drawn in, and the pass it opened in.
     last: u64,
     opened: u64,
+    /// The family the text had as the menu opened. A preview shows another one in the document
+    /// (and the menu's caller), but this one stays the current family: hovering it ends the
+    /// preview (#563).
+    current: String,
     /// The highlighted row and the query and filters it belongs to.
     highlight: Option<usize>,
     query: String,
@@ -194,6 +198,7 @@ fn list(ui: &mut Ui, state_id: egui::Id, st: &mut MenuState, current: &str, samp
         st.opened = pass;
         st.highlight = None;
         st.query.clear();
+        st.current = current.to_string();
     }
     st.last = pass;
     // For [`end_stale_preview`]: the menu is open, in this layer.
@@ -243,8 +248,9 @@ fn list(ui: &mut Ui, state_id: egui::Id, st: &mut MenuState, current: &str, samp
     });
     let enter = ui.input(|i| i.key_pressed(egui::Key::Enter));
     let db = vectorcraft_text::FontDb::global();
-    // The current family by its own name (a document may name it in Japanese: ヒラギノ角ゴシック).
-    let current = db.canonical(current, "").0;
+    // The current family by its own name (a document may name it in Japanese: ヒラギノ角ゴシック),
+    // as it was before any preview.
+    let current = db.canonical(&st.current, "").0;
     let current = current.as_str();
     let favorite = |f: &str| look.favorites.iter().any(|x| x.eq_ignore_ascii_case(f));
     // The generation is read first: fonts that load meanwhile make the next frame list them.

crates/ui-egui/src/tests_font_menu.rs

@@ -270,3 +270,61 @@ fn menus_that_only_pick_a_font_leave_the_document_alone() {
     assert_eq!(picked(&mut app, Some(FontPick::Favorite("inter".into()))), None);
     assert!(app.ui.favorite_fonts.is_empty());
 }
+
+/// #563: a preview stays on the text while the menu stays on its family. The menu's caller passes
+/// the text's font, which the preview changed; the menu keeps the font from before the preview as
+/// the current one, so it doesn't take the previewed family for it and end the preview, then
+/// preview it again, every frame (the text flickered).
+#[test]
+fn a_preview_stays_put_while_the_menu_is_still() {
+    let ctx = egui::Context::default();
+    crate::theme::install_fonts(&ctx);
+    let mut app = VectorcraftApp::new(Session::new(), Default::default());
+    app.run("file.new", json!({"width": 300, "height": 200})).unwrap();
+    let id = app.session.execute("text.create", &json!({"x": 20, "y": 50, "text": "Gagaku", "font": "Inter"})).unwrap()["id"].as_u64().unwrap();
+    app.session.execute("select.set", &json!({"ids": [id]})).unwrap();
+    let font = |app: &VectorcraftApp| match &app.session.active().unwrap().doc.node(vectorcraft_doc::NodeId(id)).unwrap().kind {
+        NodeKind::Text(t) => t.first_style().font_family,
+        _ => String::new(),
+    };
+    let revision = |app: &VectorcraftApp| app.session.active().unwrap().revision;
+    // A panel's frame: the menu shows the text's font, and what it picks is applied.
+    let frame = |app: &mut VectorcraftApp, events: Vec<egui::Event>| {
+        let input =
+            egui::RawInput { events, screen_rect: Some(egui::Rect::from_min_size(egui::Pos2::ZERO, egui::vec2(600.0, 800.0))), ..Default::default() };
+        let current = font(app);
+        let (mut at, mut picked) = (egui::Pos2::ZERO, None);
+        let mut out = ctx.run_ui(input, |ui| {
+            crate::font_menu::end_stale_preview(app, ui.ctx());
+            at = ui.next_widget_position();
+            picked = font_menu(ui, "test-steady", &current, 260.0, None, MenuLook::default());
+            if let Some(p) = picked.clone() {
+                crate::font_menu::apply(app, ui.ctx(), p);
+            }
+        });
+        out.textures_delta.clear();
+        (at, picked)
+    };
+    let (at, _) = frame(&mut app, vec![]);
+    let click = at + egui::vec2(40.0, 10.0);
+    let button = |pressed| egui::Event::PointerButton { pos: click, button: egui::PointerButton::Primary, pressed, modifiers: Default::default() };
+    frame(&mut app, vec![egui::Event::PointerMoved(click), button(true)]);
+    frame(&mut app, vec![button(false)]);
+    for _ in 0..5 {
+        frame(&mut app, vec![]);
+    }
+    let (_, picked) = frame(&mut app, key(egui::Key::ArrowDown));
+    assert!(matches!(picked, Some(FontPick::Preview(..))), "{picked:?}");
+    let previewed = font(&app);
+    assert_ne!(previewed, "Inter");
+    let rev = revision(&app);
+    for _ in 0..6 {
+        let (_, picked) = frame(&mut app, vec![]);
+        assert_eq!(picked, None, "nothing more to apply");
+    }
+    assert_eq!((font(&app), revision(&app)), (previewed, rev), "the preview stays, untouched");
+    // Back on the text's own family, the preview ends.
+    let (_, picked) = frame(&mut app, key(egui::Key::ArrowUp));
+    assert_eq!(picked, Some(FontPick::EndPreview));
+    assert_eq!(font(&app), "Inter");
+}