text item: improve partial rendering when unwrapped text changes

api/rs/slint/tests/partial_renderer.rs

@@ -681,3 +681,42 @@ fn text_alignment() {
         Some(slint::LogicalPosition { x: 10., y: 10. })
     );
 }
+
+#[test]
+fn nowrap_text_change_doesnt_change_height() {
+    slint::slint! {
+        export component Ui inherits Window {
+            in property <string> first-text: "First text";
+            out property <length> first-label-width: first-label.width;
+            out property <length> first-label-height: first-label.height;
+            background: black;
+            VerticalLayout {
+                first-label := Text {
+                    text: root.first-text;
+                }
+                Text {
+                    text: "Second text";
+                }
+            }
+        }
+    }
+
+    slint::platform::set_platform(Box::new(TestPlatform)).ok();
+    let ui = Ui::new().unwrap();
+    let window = WINDOW.with(|x| x.clone());
+    window.set_size(slint::PhysicalSize::new(180, 260));
+    ui.show().unwrap();
+    assert!(window.draw_if_needed(|renderer| {
+        do_test_render_region(renderer, 0, 0, 180, 260);
+    }));
+    assert!(!window.draw_if_needed(|_| { unreachable!() }));
+    ui.set_first_text("Hello World longer".into());
+
+    let expected_width = ui.get_first_label_width().ceil() as _;
+    let expected_height = ui.get_first_label_height().ceil() as _;
+
+    assert!(window.draw_if_needed(|renderer| {
+        do_test_render_region(renderer, 0, 0, expected_width, expected_height);
+    }));
+    assert!(!window.draw_if_needed(|_| { unreachable!() }));
+}

internal/core/items/text.rs

@@ -396,13 +396,12 @@ fn text_layout_info(
     width: Pin<&Property<LogicalLength>>,
 ) -> LayoutInfo {
     let window_inner = WindowInner::from_pub(window_adapter.window());
-    let text_string = text.text();
     let font_request = text.font_request(self_rc);
     let scale_factor = ScaleFactor::new(window_inner.scale_factor());
     let implicit_size = |max_width, text_wrap| {
         window_adapter.renderer().text_size(
             font_request.clone(),
-            text_string.as_str(),
+            text.text().as_str(),
             max_width,
             scale_factor,
             text_wrap,
@@ -435,7 +434,13 @@ fn text_layout_info(
         }
         Orientation::Vertical => {
             let h = match text.wrap() {
-                TextWrap::NoWrap => implicit_size(None, TextWrap::NoWrap).height,
+                TextWrap::NoWrap => {
+                    // For unwrapped text, the height depends only on the font, not on the text - avoid a dependency
+                    // on the text property, so that text changes result in less repaints when placed in layouts.
+                    let metrics =
+                        window_adapter.renderer().font_metrics(font_request.clone(), scale_factor);
+                    metrics.ascent - metrics.descent
+                }
                 TextWrap::WordWrap => implicit_size(Some(width.get()), TextWrap::WordWrap).height,
                 TextWrap::CharWrap => implicit_size(Some(width.get()), TextWrap::CharWrap).height,
             }