Add spacing.menu_margin for customizing menu spacing (#2036)

- Fixes https://github.com/emilk/egui/discussions/287

crates/egui/src/containers/frame.rs

@@ -75,7 +75,7 @@ impl Frame {
 
     pub fn menu(style: &Style) -> Self {
         Self {
-            inner_margin: Margin::same(1.0),
+            inner_margin: style.spacing.menu_margin,
             rounding: style.visuals.widgets.noninteractive.rounding,
             shadow: style.visuals.popup_shadow,
             fill: style.visuals.window_fill(),

crates/egui/src/style.rs

@@ -252,6 +252,9 @@ pub struct Spacing {
     /// Button size is text size plus this on each side
     pub button_padding: Vec2,
 
+    /// Horizontal and vertical margins within a menu frame.
+    pub menu_margin: Margin,
+
     /// Indent collapsing regions etc by this much.
     pub indent: f32,
 
@@ -642,6 +645,7 @@ impl Default for Spacing {
         Self {
             item_spacing: vec2(8.0, 3.0),
             window_margin: Margin::same(6.0),
+            menu_margin: Margin::same(1.0),
             button_padding: vec2(4.0, 1.0),
             indent: 18.0, // match checkbox/radio-button with `button_padding.x + icon_width + icon_spacing`
             interact_size: vec2(40.0, 18.0),
@@ -923,6 +927,7 @@ impl Spacing {
         let Self {
             item_spacing,
             window_margin,
+            menu_margin,
             button_padding,
             indent,
             interact_size,
@@ -963,12 +968,41 @@ impl Spacing {
             );
             ui.add(
                 DragValue::new(&mut window_margin.bottom)
-                    .clamp_range(margin_range)
+                    .clamp_range(margin_range.clone())
                     .prefix("bottom: "),
             );
             ui.label("Window margins y");
         });
 
+        ui.horizontal(|ui| {
+            ui.add(
+                DragValue::new(&mut menu_margin.left)
+                    .clamp_range(margin_range.clone())
+                    .prefix("left: "),
+            );
+            ui.add(
+                DragValue::new(&mut menu_margin.right)
+                    .clamp_range(margin_range.clone())
+                    .prefix("right: "),
+            );
+
+            ui.label("Menu margins x");
+        });
+
+        ui.horizontal(|ui| {
+            ui.add(
+                DragValue::new(&mut menu_margin.top)
+                    .clamp_range(margin_range.clone())
+                    .prefix("top: "),
+            );
+            ui.add(
+                DragValue::new(&mut menu_margin.bottom)
+                    .clamp_range(margin_range)
+                    .prefix("bottom: "),
+            );
+            ui.label("Menu margins y");
+        });
+
         ui.add(slider_vec2(button_padding, 0.0..=20.0, "Button padding"));
         ui.add(slider_vec2(interact_size, 4.0..=60.0, "Interact size"))
             .on_hover_text("Minimum size of an interactive widget");