test: streamline examples with quit texts

Improve some examples to provide visual feedback, i.e. for the button
exmample, with fixes to make them compilable with the `Scrollable`
element changes.

examples/elements/button.zig

@@ -7,10 +7,37 @@ const App = zterm.App(union(enum) {
 
 const log = std.log.scoped(.default);
 
-pub const Clickable = struct {
+const QuitText = struct {
+    const text = "Press ctrl+c to quit.";
+
+    pub fn element(this: *@This()) App.Element {
+        return .{ .ptr = this, .vtable = &.{ .content = content } };
+    }
+
+    pub fn content(ctx: *anyopaque, cells: []zterm.Cell, size: zterm.Size) !void {
+        _ = ctx;
+        std.debug.assert(cells.len == @as(usize, size.cols) * @as(usize, size.rows));
+
+        const row = 2;
+        const col = size.cols / 2 -| (text.len / 2);
+        const anchor = (row * size.cols) + col;
+
+        for (text, 0..) |cp, idx| {
+            cells[anchor + idx].style.fg = .white;
+            cells[anchor + idx].style.bg = .black;
+            cells[anchor + idx].cp = cp;
+
+            // NOTE: do not write over the contents of this `Container`'s `Size`
+            if (anchor + idx == cells.len - 1) break;
+        }
+    }
+};
+
+const Clickable = struct {
     const text = "Press me";
 
     queue: *App.Queue,
+    color: zterm.Color = .black,
 
     pub fn element(this: *@This()) App.Element {
         return .{
@@ -25,13 +52,20 @@ pub const Clickable = struct {
     fn handle(ctx: *anyopaque, event: App.Event) !void {
         const this: *@This() = @ptrCast(@alignCast(ctx));
         switch (event) {
-            .mouse => |mouse| this.queue.push(.{ .click = @tagName(mouse.button) }),
+            .mouse => |mouse| {
+                var value = @intFromEnum(this.color);
+                value += 1;
+                value %= 17;
+                if (value == 0) value = 1;
+                this.color = @enumFromInt(value);
+                this.queue.push(.{ .click = @tagName(mouse.button) });
+            },
             else => {},
         }
     }
 
     fn content(ctx: *anyopaque, cells: []zterm.Cell, size: zterm.Size) !void {
-        _ = ctx;
+        const this: *@This() = @ptrCast(@alignCast(ctx));
         std.debug.assert(cells.len == @as(usize, size.cols) * @as(usize, size.rows));
 
         const row = size.rows / 2 -| (text.len / 2);
@@ -39,7 +73,8 @@ pub const Clickable = struct {
         const anchor = (row * size.cols) + col;
 
         for (text, 0..) |cp, idx| {
-            cells[anchor + idx].style.fg = .black;
+            cells[anchor + idx].style.fg = this.color;
+            cells[anchor + idx].style.emphasis = &.{.bold};
             cells[anchor + idx].cp = cp;
 
             // NOTE: do not write over the contents of this `Container`'s `Size`
@@ -63,10 +98,12 @@ pub fn main() !void {
     var clickable: Clickable = .{ .queue = &app.queue };
     const element = clickable.element();
 
+    var quit_text: QuitText = .{};
+
     var container = try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .grey },
         .layout = .{ .padding = .all(5) },
-    }, .{});
+    }, quit_text.element());
     defer container.deinit();
 
     try container.append(try App.Container.init(allocator, .{ .rectangle = .{ .fill = .light_grey } }, element));