diff --git a/xpra/client/gtk3/window_base.py b/xpra/client/gtk3/window_base.py
index 0d01345155..2fd162cf37 100644
--- a/xpra/client/gtk3/window_base.py
+++ b/xpra/client/gtk3/window_base.py
@@ -1390,8 +1390,11 @@ def set_opaque_region(self, rectangles=()):
         # noinspection PyArgumentList
         r = Region()
         for rect in rectangles:
-            rect = RectangleInt(*self.srect(*rect))
-            r.union(Region(rect))
+            # "opaque-region", aka "_NET_WM_OPAQUE_REGION" is meant to use unsigned values
+            # but some applications use 0xffffffff, so we have to validate it:
+            rvalues = tuple((int(v) if v < 2**32 else -1) for v in rect)
+            rectint = RectangleInt(*self.srect(*rvalues))
+            r.union(Region(rectint))
 
         def do_set_region():
             log("set_opaque_region(%s)", r)
diff --git a/xpra/x11/models/core.py b/xpra/x11/models/core.py
index 7eca7fe434..aeb6dbbe3b 100644
--- a/xpra/x11/models/core.py
+++ b/xpra/x11/models/core.py
@@ -694,7 +694,8 @@ def _handle_opaque_region_change(self) -> None:
         v: Sequence[int] = tuple(self.prop_get("_NET_WM_OPAQUE_REGION", ["u32"]) or [])
         if OPAQUE_REGION and len(v) % 4 == 0:
             while v:
-                rectangles.append(v[:4])
+                rvalues = tuple((coord if coord < 2 ** 32 else -1) for coord in v[:4])
+                rectangles.append(rvalues)
                 v = v[4:]
         metalog("_NET_WM_OPAQUE_REGION(%s)=%s (OPAQUE_REGION=%s)", v, rectangles, OPAQUE_REGION)
         self._updateprop("opaque-region", tuple(rectangles))