Adjustable Slider rail height (emilk#4092)

Adjustable Slider rail height 


![explain20240312-2](https://github.com/emilk/egui/assets/127506429/d5edfe10-8191-44ed-8567-d9d2127ce4b8)

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

Author: 2 people

crates/egui/src/style.rs

@@ -281,6 +281,9 @@ pub struct Spacing {
     /// Default width of a [`Slider`].
     pub slider_width: f32,
 
+    /// Default rail height of a [`Slider`].
+    pub slider_rail_height: f32,
+
     /// Default (minimum) width of a [`ComboBox`](crate::ComboBox).
     pub combo_width: f32,
 
@@ -1224,6 +1227,7 @@ impl Default for Spacing {
             indent: 18.0, // match checkbox/radio-button with `button_padding.x + icon_width + icon_spacing`
             interact_size: vec2(40.0, 18.0),
             slider_width: 100.0,
+            slider_rail_height: 8.0,
             combo_width: 100.0,
             text_edit_width: 280.0,
             icon_width: 14.0,
@@ -1573,6 +1577,7 @@ impl Spacing {
             indent,
             interact_size,
             slider_width,
+            slider_rail_height,
             combo_width,
             text_edit_width,
             icon_width,
@@ -1601,6 +1606,10 @@ impl Spacing {
             ui.add(DragValue::new(slider_width).clamp_range(0.0..=1000.0));
             ui.label("Slider width");
         });
+        ui.horizontal(|ui| {
+            ui.add(DragValue::new(slider_rail_height).clamp_range(0.0..=50.0));
+            ui.label("Slider rail height");
+        });
         ui.horizontal(|ui| {
             ui.add(DragValue::new(combo_width).clamp_range(0.0..=1000.0));
             ui.label("ComboBox width");

crates/egui/src/widgets/slider.rs

@@ -680,11 +680,12 @@ impl<'a> Slider<'a> {
         if ui.is_rect_visible(response.rect) {
             let value = self.get_value();
 
-            let rail_radius = ui.painter().round_to_pixel(self.rail_radius_limit(rect));
-            let rail_rect = self.rail_rect(rect, rail_radius);
-
             let visuals = ui.style().interact(response);
             let widget_visuals = &ui.visuals().widgets;
+            let spacing = &ui.style().spacing;
+
+            let rail_radius = (spacing.slider_rail_height / 2.0).at_least(0.0);
+            let rail_rect = self.rail_rect(rect, rail_radius);
 
             ui.painter().rect_filled(
                 rail_rect,
@@ -800,13 +801,6 @@ impl<'a> Slider<'a> {
         limit / 2.5
     }
 
-    fn rail_radius_limit(&self, rect: &Rect) -> f32 {
-        match self.orientation {
-            SliderOrientation::Horizontal => (rect.height() / 4.0).at_least(2.0),
-            SliderOrientation::Vertical => (rect.width() / 4.0).at_least(2.0),
-        }
-    }
-
     fn value_ui(&mut self, ui: &mut Ui, position_range: Rangef) -> Response {
         // If [`DragValue`] is controlled from the keyboard and `step` is defined, set speed to `step`
         let change = ui.input(|input| {