add(element/scrollable): implement content provider

However this is only working for the same size as the parent container
(i.e. same `Size`). The `.resize` event for the `Container` of the
scrollable element needs to be the necessary and/or required size for
the contents (regardless of the screen viewport).

src/container.zig

@@ -11,11 +11,14 @@ const log = std.log.scoped(.container);
 
 /// Border configuration struct
 pub const Border = packed struct {
+    // corners:
     pub const rounded_border: [6]u21 = .{ '╭', '─', '╮', '│', '╰', '╯' };
     pub const squared_border: [6]u21 = .{ '┌', '─', '┐', '│', '└', '┘' };
+    // separator.line:
     pub const line: [2]u21 = .{ '│', '─' };
     pub const dotted: [2]u21 = .{ '┆', '┄' };
     pub const double: [2]u21 = .{ '║', '═' };
+
     /// Color to use for the border
     color: Color = .default,
     /// Configure the corner type to be used for the border
@@ -277,6 +280,7 @@ pub fn Container(comptime Event: type) type {
                         size.rows,
                     });
                     this.size = size;
+                    try this.element.handle(event);
 
                     if (this.elements.items.len == 0) break :resize;
 
@@ -397,6 +401,7 @@ pub fn Container(comptime Event: type) type {
 
             this.properties.border.contents(cells, this.size, this.properties.layout, @truncate(this.elements.items.len));
             this.properties.rectangle.contents(cells, this.size);
+            // NOTE: Layout has no contents to provide, hence no content method is called (or even exists in the `Layout` struct)
 
             try this.element.content(cells, this.size);