add(event): mouse event has relative position for receiving elements

src/container.zig

@@ -1,4 +1,5 @@
 const std = @import("std");
+const input = @import("input.zig");
 
 const isTaggedUnion = @import("event.zig").isTaggedUnion;
 
@@ -870,7 +871,12 @@ pub fn Container(comptime Event: type) type {
         pub fn handle(this: *@This(), event: Event) !void {
             switch (event) {
                 .mouse => |mouse| if (mouse.in(this.origin, this.size)) {
-                    try this.element.handle(event);
+                    // the element receives the mouse event with relative position
+                    std.debug.assert(mouse.x >= this.origin.x and mouse.y >= this.origin.y);
+                    var relative_mouse: input.Mouse = mouse;
+                    relative_mouse.x -= this.origin.x;
+                    relative_mouse.y -= this.origin.y;
+                    try this.element.handle(.{ .mouse = relative_mouse });
                     for (this.elements.items) |*element| try element.handle(event);
                 },
                 else => {