--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -2277,16 +2277,27 @@ nsWindow::OnExposeEvent(cairo_t *cr)
 #endif
 #endif
 
 #endif // MOZ_X11
 
     bool painted = false;
     {
       if (GetLayerManager()->GetBackendType() == LayersBackend::LAYERS_BASIC) {
+        GdkScreen *screen = gdk_window_get_screen(mGdkWindow);
+        if (GetTransparencyMode() == eTransparencyTransparent &&
+            layerBuffering == BufferMode::BUFFER_NONE &&
+            gdk_screen_is_composited(screen) &&
+            gdk_window_get_visual(mGdkWindow) ==
+            gdk_screen_get_rgba_visual(screen)) {
+          // If our draw target is unbuffered and we use an alpha channel,
+          // clear the image beforehand to ensure we don't get artifacts from a
+          // reused SHM image. See bug 1258086.
+          dt->ClearRect(Rect(boundsRect));
+        }
         AutoLayerManagerSetup setupLayerManager(this, ctx, layerBuffering);
         painted = listener->PaintWindow(this, region);
       }
     }
 
 #ifdef MOZ_X11
     // PaintWindow can Destroy us (bug 378273), avoid doing any paint
     // operations below if that happened - it will lead to XError and exit().