diff --git a/widget/gtk/nsWindow.cpp b/widget/gtk/nsWindow.cpp
index 8bc7ba6c0061..b9d216a78412 100644
--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -4237,6 +4237,7 @@ static bool IsBogusLeaveNotifyEvent(GdkWindow* aWindow,
     const auto& desktopEnv = GetDesktopEnvironmentIdentifier();
     return desktopEnv.EqualsLiteral("fluxbox") ||   // Bug 1805939 comment 0.
            desktopEnv.EqualsLiteral("blackbox") ||  // Bug 1805939 comment 32.
+           desktopEnv.EqualsLiteral("lg3d") ||      // Bug 1820405.
            StringBeginsWith(desktopEnv, "fvwm"_ns);
   }();