diff --git a/lcl/interfaces/gtk3/gtk3procs.pas b/lcl/interfaces/gtk3/gtk3procs.pas
index 9f9b5aaee6..deb9acffb9 100644
--- a/lcl/interfaces/gtk3/gtk3procs.pas
+++ b/lcl/interfaces/gtk3/gtk3procs.pas
@@ -665,10 +665,10 @@ begin
     Result := Result or MK_LBUTTON;
 
   if AState and GDK_BUTTON2_MASK <> 0 then
-    Result := Result or MK_RBUTTON;
+    Result := Result or MK_MBUTTON;
 
   if AState and GDK_BUTTON3_MASK <> 0 then
-    Result := Result or MK_MBUTTON;
+    Result := Result or MK_RBUTTON;
 
   if AState and GDK_BUTTON4_MASK <> 0 then
     Result := Result or MK_XBUTTON1;