Menus, context menus and keyboard shortcuts

A menu bar, submenus and menu items are views. A menu item's items are an
ItemsControl's, so they are patched as a list's. A context menu wraps a
view: the view and its menu are two places of the wrapper.

Keys are matched in the window's KeyDown, after the focused control had its
chance: an enabled menu item in the view whose shortcut it is, or else the
app's #:keys. Avalonia's own key bindings take an ICommand, which a Bjolang
program cannot make. app-press! presses keys for a test.

🤖 Generated with [ECA](https://eca.dev) (anthropic/claude-opus-4-8)

Co-Authored-By: eca-agent <git@eca.dev>

src/view.bjo

@@ -20,9 +20,13 @@
   ;; Constructors.
   text button text-input text-area checkbox slider progress-bar dropdown image
   list-box tabs tab
+  menu-bar submenu menu-item menu-separator
   column row grid scroll border
   ;; Wrappers.
-  keyed at paint
+  keyed at paint with-context-menu
+
+  ;; Keyboard shortcuts.
+  Key key key-gesture key-message
 
   ;; Reading one.
   view-kind view-props view-events view-children view-key)
@@ -37,6 +41,7 @@
 (type (: Kind (Union TextKind ButtonKind TextInputKind TextAreaKind CheckBoxKind
                      SliderKind ProgressKind DropdownKind ImageKind
                      ListBoxKind TabsKind TabKind
+                     MenuBarKind MenuItemKind SeparatorKind ContextKind ContextMenuKind
                      ColumnKind RowKind GridKind ScrollKind BorderKind)))
 
 ;; A property of a control. A kind takes only some of them; the app ignores
@@ -70,7 +75,8 @@
                      (: Foreground string)      ; the colour of the text
                      (: BorderColour string)
                      (: BorderWidth double)
-                     (: Radius double))))       ; how round a border's corners are
+                     (: Radius double)          ; how round a border's corners are
+                     (: Shortcut string))))     ; a menu item's keys, as "Ctrl+S"
 
 ;; An event, and the message it sends.
 (type (: (On %m) (Union (: Click %m)
@@ -124,6 +130,12 @@
   (defun (with props make c) (if (string-empty? c) props (vec-add props (make c))))
   (record-set v (props (with (with (.props v) Foreground colour) Background background))))
 
+;; The view, with a menu that opens when it is clicked with the right button.
+;; The items are made by `menu-item`, `submenu` and `menu-separator`.
+(: with-context-menu (-> (View %m) #:rest (View %m) (View %m)))
+(defun (with-context-menu v #:rest items)
+  (container ContextKind [] [v (container ContextMenuKind [] (array->vec items))]))
+
 ;; The view, in a cell of its grid. Rows and columns count from 0.
 (: at (-> int int (View %m) (#:row-span int) (#:column-span int) (View %m)))
 (defun (at row column v #:row-span 1 #:column-span 1)
@@ -261,6 +273,33 @@
 (defun (tab header content)
   (View (kind TabKind) (props [(Label header)]) (events []) (children [content]) (key None)))
 
+;; A bar of menus, made by `submenu`, for the top of the window.
+(: (-> #:rest (View %m) (View %m)))
+(defun (menu-bar #:rest menus)
+  (container MenuBarKind [] (array->vec menus)))
+
+;; A menu of items, in a menu bar or in another menu.
+(: (-> string #:rest (View %m) (View %m)))
+(defun (submenu label #:rest items)
+  (View (kind MenuItemKind) (props [(Label label)]) (events []) (children (array->vec items))
+        (key None)))
+
+;; An item that sends `msg` when it is chosen. `#:shortcut`, such as
+;; "Ctrl+S", is shown beside it, and the keys send `msg` too while the item
+;; is in the view and enabled.
+(: (-> string %m (#:shortcut string) (#:enabled bool) (View %m)))
+(defun (menu-item label msg #:shortcut "" #:enabled #t)
+  (leaf MenuItemKind
+        (if (string-empty? shortcut)
+            [(Label label) (Enabled enabled)]
+            [(Label label) (Enabled enabled) (Shortcut shortcut)])
+        [(Click msg)]))
+
+;; A line between items of a menu.
+(: (-> (View %m)))
+(defun (menu-separator)
+  (leaf SeparatorKind [] []))
+
 ;; A checkbox that is `checked?`. A click sends `(on-toggle checked)`, with
 ;; the new state.
 (: (-> string bool (-> bool %m) (#:enabled bool) (#:margin double) (View %m)))
@@ -321,3 +360,21 @@
 (: (-> (View %m) (#:width double) (#:height double) (#:margin double) (View %m)))
 (defun (scroll child #:width 0.0 #:height 0.0 #:margin 0.0)
   (container ScrollKind (with-layout [] width height margin 0.0) [child]))
+
+;; ---------------------------------------------------------------------------
+;; Keyboard shortcuts
+;; ---------------------------------------------------------------------------
+
+;; Keys, such as "Ctrl+N" or "F5", and the message they send.
+(type (: (Key %m) (Record (: gesture string) (: msg %m))))
+
+;; `msg` is sent when `gesture` is pressed in the window, and no control that
+;; has the keyboard takes the keys itself.
+(: key (-> string %m (Key %m)))
+(defun (key gesture msg) (Key (gesture gesture) (msg msg)))
+
+(: key-gesture (-> (Key %m) string))
+(defun (key-gesture k) (.gesture k))
+
+(: key-message (-> (Key %m) %m))
+(defun (key-message k) (.msg k))