Slider: move by at least the next increment when using fixed_decimals (#7066)

fixes https://github.com/emilk/egui/issues/7065

crates/egui/src/widgets/slider.rs

@@ -736,7 +736,7 @@ impl Slider<'_> {
             let prev_value = self.get_value();
             let prev_position = self.position_from_value(prev_value, position_range);
             let new_position = prev_position + ui_point_per_step * kb_step;
-            let new_value = match self.step {
+            let mut new_value = match self.step {
                 Some(step) => prev_value + (kb_step as f64 * step),
                 None if self.smart_aim => {
                     let aim_radius = 0.49 * ui_point_per_step; // Chosen so we don't include `prev_value` in the search.
@@ -747,6 +747,19 @@ impl Slider<'_> {
                 }
                 _ => self.value_from_position(new_position, position_range),
             };
+            if let Some(max_decimals) = self.max_decimals {
+                // self.set_value rounds, so ensure we reach at the least the next breakpoint
+                // note: we give it a little bit of leeway due to floating point errors. (0.1 isn't representable in binary)
+                // 'set_value' will round it to the nearest value.
+                let min_increment = 1.0 / (10.0_f64.powi(max_decimals as i32));
+                new_value = if new_value > prev_value {
+                    f64::max(new_value, prev_value + min_increment * 1.001)
+                } else if new_value < prev_value {
+                    f64::min(new_value, prev_value - min_increment * 1.001)
+                } else {
+                    new_value
+                };
+            }
             self.set_value(new_value);
         }
 

crates/egui_kittest/tests/regression_tests.rs

@@ -1,6 +1,8 @@
-use egui::accesskit::Role;
+use egui::accesskit::{self, Role};
 use egui::{Button, ComboBox, Image, Vec2, Widget as _};
-use egui_kittest::{kittest::Queryable as _, Harness, SnapshotResults};
+#[cfg(all(feature = "wgpu", feature = "snapshot"))]
+use egui_kittest::SnapshotResults;
+use egui_kittest::{kittest::Queryable as _, Harness};
 
 #[test]
 pub fn focus_should_skip_over_disabled_buttons() {
@@ -89,6 +91,7 @@ fn test_combobox() {
 
     harness.run();
 
+    #[cfg(all(feature = "wgpu", feature = "snapshot"))]
     let mut results = SnapshotResults::new();
 
     #[cfg(all(feature = "wgpu", feature = "snapshot"))]
@@ -112,3 +115,45 @@ fn test_combobox() {
     // Popup should be closed now
     assert!(harness.query_by_label("Item 2").is_none());
 }
+
+/// `https://github.com/emilk/egui/issues/7065`
+#[test]
+pub fn slider_should_move_with_fixed_decimals() {
+    let mut value: f32 = 1.0;
+
+    let mut harness = Harness::new_ui(|ui| {
+        // Movement on arrow-key is relative to slider width; make the slider wide so the movement becomes small.
+        ui.spacing_mut().slider_width = 2000.0;
+        ui.add(egui::Slider::new(&mut value, 0.1..=10.0).fixed_decimals(2));
+    });
+
+    harness.key_press(egui::Key::Tab);
+    harness.run();
+
+    let actual_slider = harness.get_by_role(accesskit::Role::SpinButton);
+    assert_eq!(actual_slider.value(), Some("1.00".to_owned()));
+
+    harness.key_press(egui::Key::ArrowRight);
+    harness.run();
+
+    let actual_slider = harness.get_by_role(accesskit::Role::SpinButton);
+    assert_eq!(actual_slider.value(), Some("1.01".to_owned()));
+
+    harness.key_press(egui::Key::ArrowRight);
+    harness.run();
+
+    let actual_slider = harness.get_by_role(accesskit::Role::SpinButton);
+    assert_eq!(actual_slider.value(), Some("1.02".to_owned()));
+
+    harness.key_press(egui::Key::ArrowLeft);
+    harness.run();
+
+    let actual_slider = harness.get_by_role(accesskit::Role::SpinButton);
+    assert_eq!(actual_slider.value(), Some("1.01".to_owned()));
+
+    harness.key_press(egui::Key::ArrowLeft);
+    harness.run();
+
+    let actual_slider = harness.get_by_role(accesskit::Role::SpinButton);
+    assert_eq!(actual_slider.value(), Some("1.00".to_owned()));
+}