Add comments about why both positional input checks are required in OsuClickableContainer

Author: peppy

osu.Game/Graphics/Containers/OsuClickableContainer.cs

@@ -17,7 +17,11 @@ public partial class OsuClickableContainer : ClickableContainer, IHasTooltip
 
         private readonly Container content = new Container { RelativeSizeAxes = Axes.Both };
 
-        public override bool ReceivePositionalInputAt(Vector2 screenSpacePos) => base.ReceivePositionalInputAt(screenSpacePos) && Content.ReceivePositionalInputAt(screenSpacePos);
+        public override bool ReceivePositionalInputAt(Vector2 screenSpacePos) =>
+            // base call is checked for cases when `OsuClickableContainer` has masking applied to it directly (ie. externally in object initialisation).
+            base.ReceivePositionalInputAt(screenSpacePos)
+            // Implementations often apply masking / edge rounding at a content level, so it's imperative to check that as well.
+            && Content.ReceivePositionalInputAt(screenSpacePos);
 
         protected override Container<Drawable> Content => content;