Don't expand widgets on hover (#7808)

* Part of https://github.com/emilk/egui/issues/5605

This changes the default style of egui.

The problem with "expanding" widgets is that they now want to paint
outside their own bounds, which then requires all parent UIs to have
proper margins.

It also means hovered things are no longer properly aligned with every
other widget.

crates/egui/src/style.rs

@@ -1234,6 +1234,11 @@ pub struct WidgetVisuals {
     pub fg_stroke: Stroke,
 
     /// Make the frame this much larger.
+    ///
+    /// The problem with "expanding" widgets is that they now want to paint outside their own bounds,
+    /// which then requires all parent UIs to have proper margins.
+    ///
+    /// It also means hovered things are no longer properly aligned with every other widget.
     pub expansion: f32,
 }
 
@@ -1568,7 +1573,7 @@ impl Widgets {
                 bg_stroke: Stroke::new(1.0, Color32::from_gray(150)), // e.g. hover over window edge or button
                 fg_stroke: Stroke::new(1.5, Color32::from_gray(240)),
                 corner_radius: CornerRadius::same(3),
-                expansion: 1.0,
+                expansion: 0.0,
             },
             active: WidgetVisuals {
                 weak_bg_fill: Color32::from_gray(55),
@@ -1576,7 +1581,7 @@ impl Widgets {
                 bg_stroke: Stroke::new(1.0, Color32::WHITE),
                 fg_stroke: Stroke::new(2.0, Color32::WHITE),
                 corner_radius: CornerRadius::same(2),
-                expansion: 1.0,
+                expansion: 0.0,
             },
             open: WidgetVisuals {
                 weak_bg_fill: Color32::from_gray(45),
@@ -1613,7 +1618,7 @@ impl Widgets {
                 bg_stroke: Stroke::new(1.0, Color32::from_gray(105)), // e.g. hover over window edge or button
                 fg_stroke: Stroke::new(1.5, Color32::BLACK),
                 corner_radius: CornerRadius::same(3),
-                expansion: 1.0,
+                expansion: 0.0,
             },
             active: WidgetVisuals {
                 weak_bg_fill: Color32::from_gray(165),
@@ -1621,7 +1626,7 @@ impl Widgets {
                 bg_stroke: Stroke::new(1.0, Color32::BLACK),
                 fg_stroke: Stroke::new(2.0, Color32::BLACK),
                 corner_radius: CornerRadius::same(2),
-                expansion: 1.0,
+                expansion: 0.0,
             },
             open: WidgetVisuals {
                 weak_bg_fill: Color32::from_gray(220),

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