fix(container): rendering scrollable elements with separators

Added corresponding test cases to test the corresponding rendering of
scrollable elements.

src/element.zig

@@ -164,3 +164,156 @@ pub fn Scrollable(Event: type) type {
         }
     };
 }
+
+// TODO: nested scrollable `Container`s?'
+// TODO: reaction only for when the event is actually pushed to the corresponding `Container` rendered container
+
+test "scrollable vertical" {
+    const event = @import("event.zig");
+    const testing = @import("testing.zig");
+
+    const size: Size = .{
+        .rows = 20,
+        .cols = 30,
+    };
+
+    var box: Container(event.SystemEvent) = try .init(std.testing.allocator, .{
+        .border = .{
+            .sides = .all,
+            .color = .red,
+        },
+        .layout = .{
+            .separator = .{
+                .enabled = true,
+                .color = .red,
+            },
+            .direction = .vertical,
+            .padding = .all(1),
+        },
+        .min_size = .{ .rows = size.rows + 15 },
+    }, .{});
+    try box.append(try .init(std.testing.allocator, .{
+        .rectangle = .{ .fill = .grey },
+    }, .{}));
+    try box.append(try .init(std.testing.allocator, .{
+        .rectangle = .{ .fill = .grey },
+    }, .{}));
+    defer box.deinit();
+
+    var scrollable: Scrollable(event.SystemEvent) = .init(box);
+
+    var container: Container(event.SystemEvent) = try .init(std.testing.allocator, .{
+        .border = .{
+            .color = .green,
+            .sides = .vertical,
+        },
+    }, scrollable.element());
+    defer container.deinit();
+
+    const allocator = std.testing.allocator;
+    var renderer: testing.Renderer = .init(allocator, size);
+    defer renderer.deinit();
+
+    try container.handle(.{ .resize = size });
+    try renderer.render(Container(event.SystemEvent), &container);
+    try testing.expectEqualCells(renderer.size, @import("test/element/scrollable.vertical.top.zon"), renderer.screen);
+
+    // scroll down 15 times (exactly to the end)
+    for (0..15) |_| try container.handle(.{
+        .mouse = .{
+            .button = .wheel_down,
+            .kind = .press,
+            .col = 5,
+            .row = 5,
+        },
+    });
+    try renderer.render(Container(event.SystemEvent), &container);
+    try testing.expectEqualCells(renderer.size, @import("test/element/scrollable.vertical.bottom.zon"), renderer.screen);
+
+    // further scrolling down will not change anything
+    try container.handle(.{
+        .mouse = .{
+            .button = .wheel_down,
+            .kind = .press,
+            .col = 5,
+            .row = 5,
+        },
+    });
+    try renderer.render(Container(event.SystemEvent), &container);
+    try testing.expectEqualCells(renderer.size, @import("test/element/scrollable.vertical.bottom.zon"), renderer.screen);
+}
+
+test "scrollable horizontal" {
+    const event = @import("event.zig");
+    const testing = @import("testing.zig");
+
+    const size: Size = .{
+        .rows = 20,
+        .cols = 30,
+    };
+
+    var box: Container(event.SystemEvent) = try .init(std.testing.allocator, .{
+        .border = .{
+            .sides = .all,
+            .color = .red,
+        },
+        .layout = .{
+            .separator = .{
+                .enabled = true,
+                .color = .red,
+            },
+            .direction = .horizontal,
+            .padding = .all(1),
+        },
+        .min_size = .{ .cols = size.cols + 15 },
+    }, .{});
+    try box.append(try .init(std.testing.allocator, .{
+        .rectangle = .{ .fill = .grey },
+    }, .{}));
+    try box.append(try .init(std.testing.allocator, .{
+        .rectangle = .{ .fill = .grey },
+    }, .{}));
+    defer box.deinit();
+
+    var scrollable: Scrollable(event.SystemEvent) = .init(box);
+
+    var container: Container(event.SystemEvent) = try .init(std.testing.allocator, .{
+        .border = .{
+            .color = .green,
+            .sides = .horizontal,
+        },
+    }, scrollable.element());
+    defer container.deinit();
+
+    const allocator = std.testing.allocator;
+    var renderer: testing.Renderer = .init(allocator, size);
+    defer renderer.deinit();
+
+    try container.handle(.{ .resize = size });
+    try renderer.render(Container(event.SystemEvent), &container);
+    try testing.expectEqualCells(renderer.size, @import("test/element/scrollable.horizontal.left.zon"), renderer.screen);
+
+    // scroll right 15 times (exactly to the end)
+    for (0..15) |_| try container.handle(.{
+        .mouse = .{
+            .button = .wheel_right,
+            .kind = .press,
+            .col = 5,
+            .row = 5,
+        },
+    });
+    try renderer.render(Container(event.SystemEvent), &container);
+    try testing.expectEqualCells(renderer.size, @import("test/element/scrollable.horizontal.right.zon"), renderer.screen);
+
+    // further scrolling right will not change anything
+    try container.handle(.{
+        .mouse = .{
+            .button = .wheel_right,
+            .kind = .press,
+            .col = 5,
+            .row = 5,
+        },
+    });
+    try renderer.render(Container(event.SystemEvent), &container);
+    try testing.expectEqualCells(renderer.size, @import("test/element/scrollable.horizontal.right.zon"), renderer.screen);
+}