Allow dropping window onto toplevel list area

Fixes https://github.com/pop-os/cosmic-workspaces-epoch/issues/53.

src/dnd.rs

@@ -63,6 +63,7 @@ impl TryFrom<(Vec<u8>, std::string::String)> for DragToplevel {
 #[repr(u8)]
 pub enum DropTarget {
     WorkspaceSidebarEntry(ZcosmicWorkspaceHandleV1, wl_output::WlOutput),
+    OutputToplevels(ZcosmicWorkspaceHandleV1, wl_output::WlOutput),
 }
 
 impl DropTarget {
@@ -76,6 +77,10 @@ impl DropTarget {
                 let id = workspace.id().protocol_id();
                 (u64::from(discriminant) << 32) | u64::from(id)
             }
+            Self::OutputToplevels(_workspace, output) => {
+                let id = output.id().protocol_id();
+                (u64::from(discriminant) << 32) | u64::from(id)
+            }
         }
     }
 }

src/main.rs

@@ -484,14 +484,18 @@ impl Application for App {
             }
             Msg::DndWorkspaceDrop(_toplevel) => {
                 if let Some((DragSurface::Toplevel(handle), _)) = &self.drag_surface {
-                    if let Some(DropTarget::WorkspaceSidebarEntry(workspace, output)) =
+                    match self.drop_target.take() {
-                        self.drop_target.take()
+                        Some(
-                    {
+                            DropTarget::WorkspaceSidebarEntry(workspace, output)
-                        self.send_wayland_cmd(backend::Cmd::MoveToplevelToWorkspace(
+                            | DropTarget::OutputToplevels(workspace, output),
-                            handle.clone(),
+                        ) => {
-                            workspace,
+                            self.send_wayland_cmd(backend::Cmd::MoveToplevelToWorkspace(
-                            output,
+                                handle.clone(),
-                        ));
+                                workspace,
+                                output,
+                            ));
+                        }
+                        None => {}
                     }
                 }
             }

src/view/mod.rs

@@ -78,6 +78,20 @@ pub(crate) fn layer_surface<'a>(
         layout,
         drag_toplevel,
     );
+    // TODO multiple active workspaces? Not currently supported by cosmic.
+    let first_active_workspace = app
+        .workspaces
+        .iter()
+        .find(|i| i.outputs.contains(&surface.output) && i.is_active);
+    let toplevels = if let Some(workspace) = first_active_workspace {
+        toplevel_dnd_destination(
+            DropTarget::OutputToplevels(workspace.handle.clone(), surface.output.clone()),
+            toplevels,
+        )
+    } else {
+        // Shouldn't happen, but no drag destination if no active workspace found for output
+        cosmic::Element::from(toplevels)
+    };
     let container = match layout {
         WorkspaceLayout::Vertical => widget::layer_container(
             row![sidebar, toplevels]