docking: anchor an outer edge drop on the area it shows

The edge band and window.panel.dock to an outer edge anchor on the canvas only where the edge area narrows to the canvas column, else on a panel that is not protected, so tree order no longer decides the frame. The renderer tries the drop and offers the band only when it wraps the panels the band shows. A top (bottom) edge also narrows a split of rows to its first (last) row when only that row holds the canvas. to_actions takes the permissions, and FloatSpace records side_edge_zones.

docs/ui-docking.md

@@ -266,9 +266,16 @@ A flyout fits its content the same way, without the title bar.
   edge band is offered only when every frame keeps its minimum.
 - The edge area is the root, or the part between the docks. Without `side_edge_zones` it also
   narrows to the canvas column: where neither side of a side-by-side split is a dock and only
-  one side holds a protected panel, that side. Side panels in split frames then get no band
-  across the whole window, and the drop wraps only the canvas column. A drop on the band
-  anchors on the canvas, so `Placement::Edge` narrows the same way when applied.
+  one side holds a protected panel, that side. The top (bottom) band also narrows a split of
+  rows to its first (last) row when only that row holds the canvas, so a full-width row below
+  the canvas keeps the bottom band. Side panels in split frames then get no band across the
+  whole window, and the drop wraps only the canvas column. A drop on a narrowed band anchors
+  on the canvas, so `Placement::Edge` narrows the same way when applied; on a band that is not
+  narrowed it anchors on a panel that is not protected. The renderer tries the drop with its
+  own permissions and offers the band only when the drop wraps the panels the band shows.
+  `window.panel.dock` to an outer edge picks its anchor the same way, from the permissions
+  `apply` (or `to_actions`) gets and the area's `FloatSpace` (`with_side_edge_zones` before it
+  has drawn).
 - `DockStyle::drop_edges` (a `Sides`, all four by default) lists which sides of a group that
   takes no tabs (a canvas), and of the layout's outer edge, take a drop. A canvas's whole body
   goes to the nearest listed side; with none listed it takes nothing. In the docking profile
@@ -395,7 +402,8 @@ let closed = command.apply(&mut app.workspace, style.right_to_left, &space, |pan
   it floats. `side` is `Start` or `End`: what moves becomes a new last column of that dock, or
   a new dock; when it already is that dock's last column, the command does nothing.
 - `window.panel.dock` with `left`, `right`, `top` or `bottom` and no `anchor` or `side` makes a
-  new frame on that outer edge (`Placement::Edge`). A hidden panel is shown first.
+  new frame on that outer edge (`Placement::Edge`), wrapping what a drag to that edge would
+  (see the edge area above). A hidden panel is shown first.
 - Showing a panel (`activate`, `dock` without a destination, `ui.dock` `open`) restores a hidden
   panel's place; a panel never laid out opens as `Open` without an anchor does.
 - `window.panel.float` without a rect floats what it moves at the size a drag floats it at (see