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/input.zig

@@ -7,7 +7,33 @@ const App = zterm.App(union(enum) {
 
 const log = std.log.scoped(.default);
 
-pub const InputField = 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 InputField = struct {
     input: std.ArrayList(u21),
     queue: *App.Queue,
 
@@ -83,12 +109,14 @@ pub fn main() !void {
     var input_field: InputField = .init(allocator, &app.queue);
     defer input_field.deinit();
 
+    var quit_text: QuitText = .{};
+
     const element = input_field.element();
 
     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));