feat(container): negative layout padding

Negative paddings use the current size to calculate the padding from
the opposite orientation. For a given dimension (horizontal or vertical)
if the size is `30` a padding of `5` would be equivalent to a padding
of `-25`. This enables to describe the size of the container using
the padding property of the `Layout` and gives users more freedom to
describe different layouts.

src/container.zig

@@ -195,6 +195,33 @@ pub const Rectangle = packed struct {
         }, &container, @import("test/container/rectangle_with_parent_padding.zon"));
     }
 
+    test "fill color padding to show parent fill (negative padding)" {
+        const event = @import("event.zig");
+        const testing = @import("testing.zig");
+
+        var container: Container(event.SystemEvent) = try .init(std.testing.allocator, .{
+            .layout = .{
+                .padding = .{
+                    .top = -18,
+                    .bottom = -18,
+                    .left = -28,
+                    .right = -28,
+                },
+            },
+            .rectangle = .{ .fill = .green },
+        }, .{});
+        try container.append(try .init(std.testing.allocator, .{
+            .rectangle = .{ .fill = .white },
+        }, .{}));
+        try container.append(try .init(std.testing.allocator, .{}, .{}));
+        defer container.deinit();
+
+        try testing.expectContainerScreen(.{
+            .y = 20,
+            .x = 30,
+        }, &container, @import("test/container/rectangle_with_parent_padding.zon"));
+    }
+
     test "fill color spacer with padding" {
         const event = @import("event.zig");
         const testing = @import("testing.zig");
@@ -293,23 +320,23 @@ pub const Layout = packed struct {
     direction: enum(u1) { horizontal, vertical } = .horizontal,
     /// Padding outside of the child elements
     padding: packed struct {
-        top: u16 = 0,
-        bottom: u16 = 0,
-        left: u16 = 0,
-        right: u16 = 0,
+        top: i16 = 0,
+        bottom: i16 = 0,
+        left: i16 = 0,
+        right: i16 = 0,
 
         /// Create a padding with equivalent padding in all four directions.
-        pub fn all(padding: u16) @This() {
+        pub fn all(padding: i16) @This() {
             return .{ .top = padding, .bottom = padding, .left = padding, .right = padding };
         }
 
         /// Create a padding with equivalent padding in the left and right directions; others directions remain the default value.
-        pub fn horizontal(padding: u16) @This() {
+        pub fn horizontal(padding: i16) @This() {
             return .{ .left = padding, .right = padding };
         }
 
         /// Create a padding with equivalent padding in the top and bottom directions; others directions remain the default value.
-        pub fn vertical(padding: u16) @This() {
+        pub fn vertical(padding: i16) @This() {
             return .{ .top = padding, .bottom = padding };
         }
     } = .{},
@@ -326,6 +353,11 @@ pub const Layout = packed struct {
         } = .line,
     } = .{},
 
+    /// Calculate the absolute offset for the provided `padding` if it is negative to get the absolute padding for the given `size`.
+    pub fn getAbsolutePadding(padding: i16, size: u16) u16 {
+        return if (padding >= 0) @intCast(padding) else size -| @as(u16, @intCast(-padding));
+    }
+
     pub fn content(this: @This(), comptime C: type, cells: []Cell, origin: Point, size: Point, children: []const C) void {
         assert(cells.len == @as(usize, size.x) * @as(usize, size.y));
 
@@ -609,8 +641,8 @@ pub fn Container(comptime Event: type) type {
             this.element.reposition(origin);
 
             var offset = origin.add(.{
-                .x = layout.padding.left,
-                .y = layout.padding.top,
+                .x = Layout.getAbsolutePadding(layout.padding.left, this.size.x),
+                .y = Layout.getAbsolutePadding(layout.padding.top, this.size.y),
             });
 
             const sides = this.properties.border.sides;
@@ -642,8 +674,8 @@ pub fn Container(comptime Event: type) type {
             };
 
             if (this.elements.items.len > 0) switch (layout.direction) {
-                .horizontal => size.x += layout.padding.left + layout.padding.right,
-                .vertical => size.y += layout.padding.top + layout.padding.bottom,
+                .horizontal => size.x += Layout.getAbsolutePadding(layout.padding.left, this.size.x) + Layout.getAbsolutePadding(layout.padding.right, this.size.x),
+                .vertical => size.y += Layout.getAbsolutePadding(layout.padding.top, this.size.y) + Layout.getAbsolutePadding(layout.padding.bottom, this.size.y),
             };
 
             const sides = this.properties.border.sides;
@@ -691,16 +723,16 @@ pub fn Container(comptime Event: type) type {
         fn grow_resize(this: *@This(), max_size: Point) void {
             const layout = this.properties.layout;
             var remainder = switch (layout.direction) {
-                .horizontal => max_size.x -| (layout.padding.left + layout.padding.right),
-                .vertical => max_size.y -| (layout.padding.top + layout.padding.bottom),
+                .horizontal => max_size.x -| (Layout.getAbsolutePadding(layout.padding.left, this.size.x) + Layout.getAbsolutePadding(layout.padding.right, this.size.x)),
+                .vertical => max_size.y -| (Layout.getAbsolutePadding(layout.padding.top, this.size.y) + Layout.getAbsolutePadding(layout.padding.bottom, this.size.y)),
             };
             remainder -|= layout.gap * @as(u16, @truncate(this.elements.items.len -| 1));
 
             if (layout.separator.enabled) remainder -|= @as(u16, @truncate(this.elements.items.len -| 1));
 
             var available = switch (layout.direction) {
-                .horizontal => max_size.y -| (layout.padding.top + layout.padding.bottom),
-                .vertical => max_size.x -| (layout.padding.left + layout.padding.right),
+                .horizontal => max_size.y -| (Layout.getAbsolutePadding(layout.padding.top, this.size.y) + Layout.getAbsolutePadding(layout.padding.bottom, this.size.y)),
+                .vertical => max_size.x -| (Layout.getAbsolutePadding(layout.padding.left, this.size.x) + Layout.getAbsolutePadding(layout.padding.right, this.size.x)),
             };
 
             const sides = this.properties.border.sides;
@@ -836,6 +868,7 @@ pub fn Container(comptime Event: type) type {
 
         pub fn resize(this: *@This(), size: Point) void {
             // NOTE assume that this function is only called for the root `Container`
+            this.size = size;
             const fit_size = this.fit_resize();
             // if (fit_size.y > size.y or fit_size.x > size.x) @panic("error: cannot render in available space");
             switch (this.properties.size.grow) {