Grow the editor's canvas to reach widgets below its edge

A widget that fell off the panel, after the canvas was made smaller, was
drawn under the clip and could only be re-packed with Reflow. Editing, the
canvas now extends to the lowest widget and scrolls, with the panel's own
edge dashed and the area past it hatched, so the widget can be dragged
back. The scale stays the panel's, and view mode is unchanged.

docs/interface/dashboards.md

@@ -16,6 +16,10 @@ Edits stay in a draft; paired screens show the published version. On phones,
 tiles stack in one column and dragging is disabled, but settings remain editable.
 In edit mode, ⌘Z / Ctrl-Z undoes an edit and ⌘⇧Z / Ctrl-Shift-Z redoes it.
 
+A widget can end up below the canvas, for example after the canvas was made
+smaller. In edit mode the canvas then grows to reach it, under a dashed line
+where the panel ends. Scroll down and drag it back, or use **Reflow**.
+
 Controls can publish real values while you edit, so choose their targets with
 care. Your new Switch may already be flipping something!