add(PopupMenu): menu to enable space key binding groupings

Some minor layout changes of the application

src/widget/PopupMenu.zig

@@ -0,0 +1,80 @@
+//! Pop-up Menu widget to show the available keybindings
+
+const std = @import("std");
+const vaxis = @import("vaxis");
+
+const widget = @import("../widget.zig");
+
+const Event = widget.Event;
+
+allocator: std.mem.Allocator = undefined,
+unicode: *const vaxis.Unicode = undefined,
+
+pub fn init(allocator: std.mem.Allocator, unicode: *const vaxis.Unicode) @This() {
+    return .{
+        .allocator = allocator,
+        .unicode = unicode,
+    };
+}
+
+pub fn deinit(this: *@This()) void {
+    this.* = undefined;
+}
+
+/// Update loop for a given widget to react to the provided `Event`. It may
+/// change its internal state, update variables, react to user input, etc.
+pub fn update(this: *@This(), event: Event) ?Event {
+    _ = this;
+    switch (event) {
+        .key_press => |key| {
+            if (key.matches('a', .{})) {
+                // About
+                return .{ .path = "./doc/about.md" };
+            }
+            if (key.matches('h', .{})) {
+                // Home
+                return .{ .path = "./doc/home.md" };
+            }
+        },
+        else => {},
+    }
+    return null;
+}
+
+/// Draw a given widget using the provided `vaxis.Window`. The window controls
+/// the dimension one widget may take on the screen. The widget itself has no
+/// control over this.
+pub fn draw(this: *@This(), win: vaxis.Window) void {
+    var view = vaxis.widgets.View.init(this.allocator, this.unicode, .{ .width = win.width, .height = win.height }) catch @panic("OOM");
+    defer view.deinit();
+
+    const msg =
+        \\# Goto
+        \\
+        \\*a* about
+        \\*h* home
+    ;
+    var col: usize = 0;
+    var row: usize = 0;
+    for (msg, 0..) |_, i| {
+        const cell: vaxis.Cell = .{
+            // each cell takes a _grapheme_ as opposed to a single
+            // codepoint. This allows Vaxis to handle emoji properly,
+            // particularly with terminals that the Unicode Core extension
+            // (IE Mode 2027)
+            .char = .{ .grapheme = msg[i .. i + 1] },
+            .style = .{
+                .fg = .{ .index = 6 },
+                .bold = true,
+            },
+        };
+        view.writeCell(col, row, cell);
+        if (std.mem.eql(u8, cell.char.grapheme, "\n")) {
+            col = 0;
+            row += 1;
+        } else {
+            col += 1;
+        }
+    }
+    view.draw(win, .{});
+}