MarkdownViewer does not respond to the Select All shortcut

Pressing the Select All shortcut (Cmd+A on macOS, Ctrl+A on Windows/Linux) does nothing when a MarkdownViewer has focus, even though the same shortcut works in text fields and pointer selection of the rendered Markdown works. The Standard Edit menu's Select All (and Copy) actions also ignore a focused MarkdownViewer, because the shared standard-menu dispatcher recognises text fields, list boxes, and tree views but not MarkdownViewer targets. Users cannot use the platform shortcuts or the Edit menu to select or copy rendered Markdown text.