use the actual content size for interaction

crates/egui/src/containers/scroll_area.rs

@@ -473,6 +473,9 @@ struct Prepared {
     scrolling_enabled: bool,
     stick_to_end: Vec2b,
     animated: bool,
+
+    /// Id of the drag area. We store this until we know the size of the content to interact with.
+    drag_area_id: Id,
 }
 
 impl ScrollArea {
@@ -585,15 +588,18 @@ impl ScrollArea {
         let viewport = Rect::from_min_size(Pos2::ZERO + state.offset, inner_size);
         let dt = ui.input(|i| i.stable_dt).at_most(0.1);
 
+        let drag_area_id = id.with("area");
+
         if (scrolling_enabled && drag_to_scroll)
             && (state.content_is_too_large[0] || state.content_is_too_large[1])
         {
             // Drag contents to scroll (for touch screens mostly).
             // We must do this BEFORE adding content to the `ScrollArea`,
             // or we will steal input from the widgets we contain.
-            let content_response = ui.interact(inner_rect, id.with("area"), Sense::drag());
+            // This response is not available on the first frame.
+            let content_response_option = ui.ctx().read_response(drag_area_id);
 
-            if content_response.dragged() {
+            if content_response_option.map(|response| response.dragged()) == Some(true) {
                 for d in 0..2 {
                     if scroll_enabled[d] {
                         ui.input(|input| {
@@ -671,6 +677,7 @@ impl ScrollArea {
             scrolling_enabled,
             stick_to_end,
             animated,
+            drag_area_id,
         }
     }
 
@@ -783,6 +790,7 @@ impl Prepared {
             scrolling_enabled,
             stick_to_end,
             animated,
+            drag_area_id,
         } = self;
 
         let content_size = content_ui.min_size();
@@ -874,6 +882,10 @@ impl Prepared {
             Rect::from_min_size(inner_rect.min, inner_size)
         };
 
+        // At this point we know the actual size of the area we can interact with, so we do the interaction
+        // here. The result is ignored and read at the start of the frame.
+        let _ = ui.interact(inner_rect, drag_area_id, Sense::drag());
+
         let outer_rect = Rect::from_min_size(inner_rect.min, inner_rect.size() + current_bar_use);
 
         let content_is_too_large = Vec2b::new(