diff --git a/lcl/interfaces/gtk/gtkwinapi.inc b/lcl/interfaces/gtk/gtkwinapi.inc
index a96b34b059..add2e86b01 100644
--- a/lcl/interfaces/gtk/gtkwinapi.inc
+++ b/lcl/interfaces/gtk/gtkwinapi.inc
@@ -7586,7 +7586,16 @@ begin
   //DebugLn(Format('trace:< [TGtkWidgetSet.Rectangle] DC:0x%x, X1:%d, Y1:%d, X2:%d, Y2:%d', [DC, X1, Y1, X2, Y2]));
 end;
 
-// Determines if the specified rectangle is within the boundaries of a region.
+{------------------------------------------------------------------------------
+  Function: RectInRegion
+  Params:   RGN: HRGN; ARect: TRect
+  Returns:  True if any part of the specified rectangle lies within the
+            boundaries of the region.
+
+  Determines whether any part of the specified rectangle is within the boundaries
+  of a region.
+
+ ------------------------------------------------------------------------------}
 function TGtkWidgetSet.RectInRegion(RGN: HRGN; ARect: TRect): Boolean;
 var
   AGdkRect: TGdkRectangle;
@@ -7594,7 +7603,7 @@ begin
   //todo: sanity checks for valid handle etc.
   AGdkRect := GdkRectFromRect(ARect);
   Result := gdk_region_rect_in({%H-}PGdiObject(RGN)^.GDIRegionObject, @AGdkRect)
-             = GDK_OVERLAP_RECTANGLE_IN;
+             <> GDK_OVERLAP_RECTANGLE_OUT;
 end;
 
 {------------------------------------------------------------------------------
diff --git a/lcl/interfaces/gtk2/gtk2winapi.inc b/lcl/interfaces/gtk2/gtk2winapi.inc
index bb68529152..f3ef15f6b9 100644
--- a/lcl/interfaces/gtk2/gtk2winapi.inc
+++ b/lcl/interfaces/gtk2/gtk2winapi.inc
@@ -7055,7 +7055,16 @@ begin
   {$IFDEF DebugGDKTraps}EndGDKErrorTrap;{$ENDIF}
 end;
 
-// Determines if the specified rectangle is within the boundaries of a region.
+{------------------------------------------------------------------------------
+  Function: RectInRegion
+  Params:   RGN: HRGN; ARect: TRect
+  Returns:  True if any part of the specified rectangle lies within the
+            boundaries of the region.
+
+  Determines whether any part of the specified rectangle is within the boundaries
+  of a region.
+
+ ------------------------------------------------------------------------------}
 function TGtk2WidgetSet.RectInRegion(RGN: HRGN; ARect: TRect): Boolean;
 var
   AGdkRect: TGdkRectangle;
@@ -7063,7 +7072,7 @@ begin
   //todo: sanity checks for valid handle etc.
   AGdkRect := GdkRectFromRect(ARect);
   Result := gdk_region_rect_in({%H-}PGdiObject(RGN)^.GDIRegionObject, @AGdkRect)
-             = GDK_OVERLAP_RECTANGLE_IN;
+             <> GDK_OVERLAP_RECTANGLE_OUT;
 end;
 
 {------------------------------------------------------------------------------