Store Margin using i8 to reduce its size (#5567)

Adds `Marginf` to fill the previous niche.

This is all in a pursuit to shrink the sizes of often-used structs, to
improve performance (less cache misses, less memcpy:s, etc).

* On the path towards https://github.com/emilk/egui/issues/4019

crates/egui/src/containers/frame.rs

@@ -4,7 +4,7 @@ use crate::{
     epaint, layers::ShapeIdx, InnerResponse, Response, Sense, Style, Ui, UiBuilder, UiKind,
     UiStackInfo,
 };
-use epaint::{Color32, Margin, Rect, Rounding, Shadow, Shape, Stroke};
+use epaint::{Color32, Margin, Marginf, Rect, Rounding, Shadow, Shape, Stroke};
 
 /// Add a background, frame and/or margin to a rectangular background of a [`Ui`].
 ///
@@ -73,6 +73,18 @@ pub struct Frame {
     pub stroke: Stroke,
 }
 
+#[test]
+fn frame_size() {
+    assert_eq!(
+        std::mem::size_of::<Frame>(), 44,
+        "Frame changed size! If it shrank - good! Update this test. If it grew - bad! Try to find a way to avoid it."
+    );
+    assert!(
+        std::mem::size_of::<Frame>() <= 64,
+        "Frame is getting way too big!"
+    );
+}
+
 impl Frame {
     pub fn none() -> Self {
         Self::default()
@@ -81,7 +93,7 @@ impl Frame {
     /// For when you want to group a few widgets together within a frame.
     pub fn group(style: &Style) -> Self {
         Self {
-            inner_margin: Margin::same(6.0), // same and symmetric looks best in corners when nesting groups
+            inner_margin: Margin::same(6), // same and symmetric looks best in corners when nesting groups
             rounding: style.visuals.widgets.noninteractive.rounding,
             stroke: style.visuals.widgets.noninteractive.bg_stroke,
             ..Default::default()
@@ -90,7 +102,7 @@ impl Frame {
 
     pub fn side_top_panel(style: &Style) -> Self {
         Self {
-            inner_margin: Margin::symmetric(8.0, 2.0),
+            inner_margin: Margin::symmetric(8, 2),
             fill: style.visuals.panel_fill,
             ..Default::default()
         }
@@ -98,7 +110,7 @@ impl Frame {
 
     pub fn central_panel(style: &Style) -> Self {
         Self {
-            inner_margin: Margin::same(8.0),
+            inner_margin: Margin::same(8),
             fill: style.visuals.panel_fill,
             ..Default::default()
         }
@@ -143,7 +155,7 @@ impl Frame {
     /// and in dark mode this will be very dark.
     pub fn canvas(style: &Style) -> Self {
         Self {
-            inner_margin: Margin::same(2.0),
+            inner_margin: Margin::same(2),
             rounding: style.visuals.widgets.noninteractive.rounding,
             fill: style.visuals.extreme_bg_color,
             stroke: style.visuals.window_stroke(),
@@ -213,10 +225,10 @@ impl Frame {
 }
 
 impl Frame {
-    /// inner margin plus outer margin.
+    /// Inner margin plus outer margin.
     #[inline]
-    pub fn total_margin(&self) -> Margin {
-        self.inner_margin + self.outer_margin
+    pub fn total_margin(&self) -> Marginf {
+        Marginf::from(self.inner_margin) + Marginf::from(self.outer_margin)
     }
 }
 

crates/egui/src/containers/window.rs

@@ -486,7 +486,7 @@ impl<'open> Window<'open> {
         // Calculate roughly how much larger the window size is compared to the inner rect
         let (title_bar_height, title_content_spacing) = if with_title_bar {
             let style = ctx.style();
-            let spacing = window_margin.top + window_margin.bottom;
+            let spacing = window_margin.sum().y;
             let height = ctx.fonts(|f| title.font_height(f, &style)) + spacing;
             let half_height = (height / 2.0).round() as _;
             window_frame.rounding.ne = window_frame.rounding.ne.clamp(0, half_height);

crates/egui/src/style.rs

@@ -1238,8 +1238,8 @@ impl Default for Spacing {
     fn default() -> Self {
         Self {
             item_spacing: vec2(8.0, 3.0),
-            window_margin: Margin::same(6.0),
-            menu_margin: Margin::same(6.0),
+            window_margin: Margin::same(6),
+            menu_margin: Margin::same(6),
             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),
@@ -2371,9 +2371,17 @@ impl Widget for &mut Margin {
 
         // Apply the checkbox:
         if same {
-            *self = Margin::same((self.left + self.right + self.top + self.bottom) / 4.0);
-        } else if self.is_same() {
-            self.right *= 1.00001; // prevent collapsing into sameness
+            *self =
+                Margin::from((self.leftf() + self.rightf() + self.topf() + self.bottomf()) / 4.0);
+        } else {
+            // Make sure it is not same:
+            if self.is_same() {
+                if self.right == i8::MAX {
+                    self.right = i8::MAX - 1;
+                } else {
+                    self.right += 1;
+                }
+            }
         }
 
         response

crates/egui/src/widgets/text_edit/builder.rs

@@ -126,7 +126,7 @@ impl<'t> TextEdit<'t> {
             layouter: None,
             password: false,
             frame: true,
-            margin: Margin::symmetric(4.0, 2.0),
+            margin: Margin::symmetric(4, 2),
             multiline: true,
             interactive: true,
             desired_width: None,