Fix nested scrolling (#83)

Add scroll_delta in FrameState to fix nested scrolling.

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

egui/src/containers/scroll_area.rs

@@ -230,9 +230,19 @@ impl Prepared {
             }
         }
 
-        // TODO: check that nothing else is being interacted with
+        let max_offset = content_size.y - inner_rect.height();
         if ui.rect_contains_mouse(outer_rect) {
-            state.offset.y -= ui.input().scroll_delta.y;
+            let mut frame_state = ui.ctx().frame_state();
+            let scroll_delta = frame_state.scroll_delta;
+
+            let scrolling_up = state.offset.y > 0.0 && scroll_delta.y > 0.0;
+            let scrolling_down = state.offset.y < max_offset && scroll_delta.y < 0.0;
+
+            if scrolling_up || scrolling_down {
+                state.offset.y -= scroll_delta.y;
+                // Clear scroll delta so no parent scroll will use it.
+                frame_state.scroll_delta = Vec2::zero();
+            }
         }
 
         let show_scroll_this_frame = content_is_too_small || always_show_scroll;
@@ -286,7 +296,7 @@ impl Prepared {
             }
 
             state.offset.y = state.offset.y.max(0.0);
-            state.offset.y = state.offset.y.min(content_size.y - inner_rect.height());
+            state.offset.y = state.offset.y.min(max_offset);
 
             // Avoid frame-delay by calculating a new handle rect:
             let mut handle_rect = Rect::from_min_max(

egui/src/context.rs

@@ -38,6 +38,7 @@ pub(crate) struct FrameState {
 
     /// How much space is used by panels.
     used_by_panels: Rect,
+    pub(crate) scroll_delta: Vec2,
     pub(crate) scroll_target: Option<(f32, Align)>,
     // TODO: move some things from `Memory` to here
 }
@@ -48,6 +49,7 @@ impl Default for FrameState {
             available_rect: Rect::invalid(),
             unused_rect: Rect::invalid(),
             used_by_panels: Rect::invalid(),
+            scroll_delta: Vec2::zero(),
             scroll_target: None,
         }
     }
@@ -58,6 +60,7 @@ impl FrameState {
         self.available_rect = input.screen_rect();
         self.unused_rect = input.screen_rect();
         self.used_by_panels = Rect::nothing();
+        self.scroll_delta = input.scroll_delta;
         self.scroll_target = None;
     }