WorkspaceSystem: sort windows by the time they appeared on workspace

src/WindowSystem/Window.vala

@@ -10,6 +10,7 @@ public class Dock.Window : GLib.Object {
     public string app_id { get; private set; default = ""; }
     public bool has_focus { get; private set; default = false; }
     public int workspace_index { get; private set; default = 0; }
+    public int64 time_appeared_on_workspace { get; private set; default = 0; }
     public bool on_active_workspace { get; private set; default = false; }
 
     public GLib.Icon icon { get; private set; default = new GLib.ThemedIcon ("application-default-icon"); }
@@ -31,6 +32,10 @@ public class Dock.Window : GLib.Object {
             workspace_index = (int) properties["workspace-index"];
         }
 
+        if ("time-appeared-on-workspace" in properties) {
+            time_appeared_on_workspace = (int64) properties["time-appeared-on-workspace"];
+        }
+
         if ("on-active-workspace" in properties) {
             on_active_workspace = (bool) properties["on-active-workspace"];
         }

src/WorkspaceSystem/WorkspaceSystem.vala

@@ -55,6 +55,8 @@ public class Dock.WorkspaceSystem : Object {
             workspace_window_list[workspace_index].add (window);
         }
 
+
+
         // update windows in existing workspaces
         for (var i = 0; i < n_workspaces; i++) {
             Workspace workspace;
@@ -64,12 +66,18 @@ public class Dock.WorkspaceSystem : Object {
                 workspace = add_workspace ();
             }
 
+            workspace_window_list[i].sort (compare_func);
+
             workspace.windows = workspace_window_list[i];
             workspace.index = i;
             workspace.update_active_workspace ();
         }
     }
 
+    private int compare_func (Window a, Window b) {
+        return (int) (a.time_appeared_on_workspace - b.time_appeared_on_workspace);
+    }
+
     private async void sync_active_workspace () {
         foreach (var workspace in workspaces) {
             workspace.update_active_workspace ();