diff --git a/lcl/forms.pp b/lcl/forms.pp
index fe816f6e37..0c6724f667 100644
--- a/lcl/forms.pp
+++ b/lcl/forms.pp
@@ -480,6 +480,7 @@ type
     procedure WMPaint(var message: TLMPaint); message LM_PAINT;
     procedure WMShowWindow(var message: TLMShowWindow); message LM_SHOWWINDOW;
     procedure WMSize(var message: TLMSize); message LM_Size;
+    procedure WMWindowPosChanged(var Message: TLMWindowPosChanged); message LM_WINDOWPOSCHANGED;
     procedure CMBiDiModeChanged(var Message: TLMessage); message CM_BIDIMODECHANGED;
     procedure CMParentBiDiModeChanged(var Message: TLMessage); message CM_PARENTBIDIMODECHANGED;
     procedure CMAppShowBtnGlyphChanged(var Message: TLMessage); message CM_APPSHOWBTNGLYPHCHANGED;
diff --git a/lcl/include/customform.inc b/lcl/include/customform.inc
index 2fbb61a143..47a0b4adc4 100644
--- a/lcl/include/customform.inc
+++ b/lcl/include/customform.inc
@@ -656,7 +656,7 @@ begin
 
   inherited WMSize(Message);
 
-  if (Message.SizeType and not Size_SourceIsInterface) = Size_Restored then
+  if (Message.SizeType and Size_Restored)>0 then
   begin
     FRestoredLeft := Left;
     FRestoredTop := Top;
@@ -666,6 +666,23 @@ begin
   end;
 end;
 
+procedure TCustomForm.WMWindowPosChanged(var Message: TLMWindowPosChanged);
+begin
+  if (Parent=nil) and (Message.WindowPos<>nil)
+  and ((Message.WindowPos^.flags and SWP_SourceIsInterface)>0)
+  then begin
+    // this is a top level form (constraints depend on window manager)
+    // and the widgetset set a size
+    if (Message.WindowPos^.cx<>Width) or (Message.WindowPos^.cy<>Height) then
+    begin
+      // the window manager sets another size => disable autosize to prevent endless loop
+      Include(FFormState,fsDisableAutoSize);
+    end;
+  end;
+
+  inherited WMWindowPosChanged(Message);
+end;
+
 procedure TCustomForm.CMBiDiModeChanged(var Message: TLMessage);
 var
   i: Integer;
@@ -680,9 +697,11 @@ begin
   lMessage.Result := 0;
   DisableAlign;
   try
+    {$IFDEF OldAutoSize}
     if Parent<>nil then
       Parent.ReAlign
     else
+    {$ENDIF}
       AdjustSize;
     for i := 0 to ComponentCount - 1 do
     begin
diff --git a/lcl/include/wincontrol.inc b/lcl/include/wincontrol.inc
index b7b1b25130..e276271c17 100644
--- a/lcl/include/wincontrol.inc
+++ b/lcl/include/wincontrol.inc
@@ -6871,7 +6871,8 @@ begin
     ' BoundsRealized=',dbgs(FBoundsRealized),' FromIntf=',(Message.WindowPos^.flags and SWP_SourceIsInterface)>0,' ClientRectInvalid=',ClientRectNeedsInterfaceUpdate]);
   {$ENDIF}
 
-  if ((Message.WindowPos^.flags and SWP_SourceIsInterface) = 0) then begin
+  if (Message.WindowPos=nil)
+  or ((Message.WindowPos^.flags and SWP_SourceIsInterface) = 0) then begin
     inherited WMWindowPosChanged(Message);
     exit;
   end;
@@ -6889,11 +6890,11 @@ begin
   and (not (wcfClientRectNeedsUpdate in FWinControlFlags)) then
     exit;
 
-  {$IFDEF VerboseAllAutoSize}
+  { $IFDEF VerboseAllAutoSize}
   DebugLn(['TWinControl.WMWindowPosChanged CHANGED BY INTF ',dbgsname(Self),' Message=',NewLeft,',',NewTop,',',NewWidth,'x',NewHeight,
     ' BoundsRealized=',dbgs(FBoundsRealized),
     ' wcfClientRectNeedsUpdate=',wcfClientRectNeedsUpdate in FWinControlFlags]);
-  {$ENDIF}
+  { $ENDIF}
   FBoundsRealized := NewBoundsRealized;
   {$IFNDEF OldAutoSize}
   //DebugLn(['TWinControl.WMSize ',DbgSName(Self),' phases=',dbgs(AutoSizePhases)]);
@@ -8173,13 +8174,13 @@ begin
     ' -> NewBounds=',dbgs(NewBounds),
     ' ClientRect=',dbgs(ClientRect));
   {$ENDIF}
-  {$IFDEF VerboseBug16184}
+  { $IFDEF VerboseBug16184}
   if (Parent=nil) and (Visible) then begin
     DebugLn('[TWinControl.DoSendBoundsToInterface] A ',DbgSName(Self),
     ' OldRelBounds=',dbgs(FBoundsRealized),
     ' -> NewBounds=',dbgs(NewBounds));
   end;
-  {$ENDIF}
+  { $ENDIF}
   FBoundsRealized:=NewBounds;
   Include(FWinControlFlags,wcfBoundsRealized); // Note: set before calling widgetset, because used in WMSize
   //if Parent=nil then DebugLn(['TWinControl.DoSendBoundsToInterface ',DbgSName(Self),' ',dbgs(BoundsRect)]);
diff --git a/lcl/interfaces/gtk/gtkproc.inc b/lcl/interfaces/gtk/gtkproc.inc
index 943dd47940..fe40eca8e6 100644
--- a/lcl/interfaces/gtk/gtkproc.inc
+++ b/lcl/interfaces/gtk/gtkproc.inc
@@ -6617,10 +6617,10 @@ var
   GtkLeft, GtkTop, GtkWidth, GtkHeight: integer;
   TopLeftChanged, WidthHeightChanged, IsTopLevelWidget: boolean;
   MessageDelivered: boolean;
-  {$IFDEF OldAutoSize}
+  { $IFDEF OldAutoSize}
   SizeMsg: TLMSize;
   MoveMsg: TLMMove;
-  {$ENDIF}
+  { $ENDIF}
   PosMsg : TLMWindowPosChanged;
   MainWidget: PGtkWidget;
   FixedWidget: PGtkWidget;
@@ -6730,10 +6730,9 @@ begin
         y := GtkTop;
         cx := GtkWidth;
         cy := GtkHeight;
-        {$IFDEF OldAutoSize}
-        flags := 0;
-        {$ELSE}
-        flags := SWP_SourceIsInterface;
+        flags:=0;
+        {$IFNDEF OldAutoSize}
+       // flags := SWP_SourceIsInterface;
         {$ENDIF}
       end;
       MessageDelivered := DeliverMessage(LCLControl, PosMsg) = 0;
@@ -6746,12 +6745,12 @@ begin
     UpdateLCLSize;
   end;
 
-  {$IFDEF OldAutoSize}
+  { $IFDEF OldAutoSize}
   // then send a LM_SIZE message
   if WidthHeightChanged then begin
-    {$IFDEF VerboseSizeMsg}
+    { $IFDEF VerboseSizeMsg}
     DebugLn('Send LM_SIZE To LCL ',LCLControl.Name,':',LCLControl.ClassName);
-    {$ENDIF}
+    { $ENDIF}
     with SizeMsg do
     begin
       Result := 0;
@@ -6786,9 +6785,9 @@ begin
 
   // then send a LM_MOVE message
   if TopLeftChanged then begin
-    {$IFDEF VerboseSizeMsg}
+    { $IFDEF VerboseSizeMsg}
     DebugLn('Send LM_MOVE To LCL ',LCLControl.Name,':',LCLControl.ClassName);
-    {$ENDIF}
+    { $ENDIF}
     with MoveMsg do
     begin
       Result := 0;
@@ -6800,7 +6799,7 @@ begin
     MessageDelivered := (DeliverMessage(LCLControl,  MoveMsg) = 0);
     if not MessageDelivered then exit;
   end;
-  {$ENDIF}
+  { $ENDIF}
 
   {$ifndef gtk1}
   if GtkWidgetIsA(aWidget, GTKAPIWidget_Type) and