Tweak ScrollArea fade effect (#8023)

* Follow-up to https://github.com/emilk/egui/pull/8018

crates/egui/src/containers/scroll_area.rs

@@ -1513,11 +1513,11 @@ impl Prepared {
 /// indicate that more content is available beyond the visible region.
 fn paint_fade_areas<R>(ui: &Ui, scroll_output: &ScrollAreaOutput<R>) {
     let crate::style::ScrollFadeStyle {
-        enabled,
+        strength,
         size: fade_size,
     } = ui.spacing().scroll.fade;
 
-    if !enabled {
+    if strength <= 0.0 {
         return;
     }
 
@@ -1533,7 +1533,7 @@ fn paint_fade_areas<R>(ui: &Ui, scroll_output: &ScrollAreaOutput<R>) {
 
     // Top fade: animate opacity based on how far we've scrolled down.
     if 0.0 < offset.y {
-        let t = (offset.y / fade_size).clamp(0.0, 1.0);
+        let t = (offset.y / fade_size).clamp(0.0, 1.0) * strength;
         let bg_faded = bg.gamma_multiply(t);
         let rect = Rect::from_min_max(
             paint_rect.left_top(),
@@ -1549,7 +1549,7 @@ fn paint_fade_areas<R>(ui: &Ui, scroll_output: &ScrollAreaOutput<R>) {
     // Bottom fade: animate opacity based on distance from the bottom.
     let distance_from_bottom = overflow.y - offset.y;
     if 0.0 < distance_from_bottom {
-        let t = (distance_from_bottom / fade_size).clamp(0.0, 1.0);
+        let t = (distance_from_bottom / fade_size).clamp(0.0, 1.0) * strength;
         let bg_faded = bg.gamma_multiply(t);
         let rect = Rect::from_min_max(
             pos2(paint_rect.left(), paint_rect.bottom() - fade_size),
@@ -1563,9 +1563,8 @@ fn paint_fade_areas<R>(ui: &Ui, scroll_output: &ScrollAreaOutput<R>) {
     }
 
     // Left fade: animate opacity based on how far we've scrolled right.
-
     if 0.0 < offset.x {
-        let t = (offset.x / fade_size).clamp(0.0, 1.0);
+        let t = (offset.x / fade_size).clamp(0.0, 1.0) * strength;
         let bg_faded = bg.gamma_multiply(t);
         let rect = Rect::from_min_max(
             paint_rect.left_top(),
@@ -1581,7 +1580,7 @@ fn paint_fade_areas<R>(ui: &Ui, scroll_output: &ScrollAreaOutput<R>) {
     // Right fade: animate opacity based on distance from the right edge.
     let distance_from_right = overflow.x - offset.x;
     if 0.0 < distance_from_right {
-        let t = (distance_from_right / fade_size).clamp(0.0, 1.0);
+        let t = (distance_from_right / fade_size).clamp(0.0, 1.0) * strength;
         let bg_faded = bg.gamma_multiply(t);
         let rect = Rect::from_min_max(
             pos2(paint_rect.right() - fade_size, paint_rect.top()),

crates/egui/src/style.rs

@@ -790,8 +790,10 @@ impl ScrollStyle {
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 #[cfg_attr(feature = "serde", serde(default))]
 pub struct ScrollFadeStyle {
-    /// Show fade areas?
+    /// Opacity of the fade effect at the outer edge, in 0.0-1.0.
-    pub enabled: bool,
+    ///
+    /// Set to 0.0 to disable the fade effect.
+    pub strength: f32,
 
     /// Size of the fade-area (height for vertical scrolling,
     /// width for horizontal scrolling).
@@ -801,7 +803,7 @@ pub struct ScrollFadeStyle {
 impl Default for ScrollFadeStyle {
     fn default() -> Self {
         Self {
-            enabled: true,
+            strength: 0.5,
             size: 20.0,
         }
     }
@@ -809,13 +811,14 @@ impl Default for ScrollFadeStyle {
 
 impl ScrollFadeStyle {
     pub fn ui(&mut self, ui: &mut Ui) {
-        let Self { enabled, size } = self;
+        let Self { strength, size } = self;
 
         ui.horizontal(|ui| {
-            ui.checkbox(enabled, "Fade edges");
+            ui.add(DragValue::new(strength).speed(0.01).range(0.0..=1.0));
+            ui.label("Fade strength");
         });
 
-        if *enabled {
+        if 0.0 < *strength {
             ui.horizontal(|ui| {
                 ui.add(DragValue::new(size).range(0.0..=64.0));
                 ui.label("Fade size");

crates/egui_demo_app/tests/snapshots/easymarkeditor.png

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-size 169277
+size 169664

crates/egui_demo_lib/tests/snapshots/demos/Font Book.png

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crates/egui_demo_lib/tests/snapshots/demos/Panels.png

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crates/egui_demo_lib/tests/snapshots/demos/Scrolling.png

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crates/egui_demo_lib/tests/snapshots/demos/Table.png

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crates/egui_demo_lib/tests/snapshots/demos/Tooltips.png

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crates/egui_demo_lib/tests/snapshots/demos/Window Resize Test.png

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crates/egui_kittest/tests/snapshots/test_scroll_initial.png

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crates/egui_kittest/tests/snapshots/test_scroll_scrolled.png

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tests/egui_tests/tests/snapshots/text_edit_scroll_0_focus.png

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tests/egui_tests/tests/snapshots/text_edit_scroll_1_5.png

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