diff --git a/src/InternalUtils.vala b/src/InternalUtils.vala
index ff32204e..9177d7cb 100644
--- a/src/InternalUtils.vala
+++ b/src/InternalUtils.vala
@@ -143,6 +143,9 @@ namespace Gala {
             unowned WindowClone id;
         }
 
+        /**
+         * Careful: List<TilableWindow?> windows will be modified in place and shouldn't be used afterwards.
+         */
 #if HAS_MUTTER45
         public static List<TilableWindow?> calculate_grid_placement (Mtk.Rectangle area, List<TilableWindow?> windows) {
 #else
@@ -172,9 +175,8 @@ namespace Gala {
             }
 
             // Assign each window to the closest available slot
-            var tmplist = windows.copy ();
-            while (tmplist.length () > 0) {
-                unowned List<unowned TilableWindow?> link = tmplist.nth (0);
+            while (windows.length () > 0) {
+                unowned List<TilableWindow?> link = windows.nth (0);
                 var window = link.data;
                 var rect = window.rect;
 
@@ -207,9 +209,9 @@ namespace Gala {
                     continue;
 
                 if (taken_slots[slot_candidate] != null)
-                    tmplist.prepend (taken_slots[slot_candidate]);
+                    windows.prepend (taken_slots[slot_candidate]);
 
-                tmplist.remove_link (link);
+                windows.remove_link (link);
                 taken_slots[slot_candidate] = window;
             }