diff --git a/lcl/controls.pp b/lcl/controls.pp
index e28d7df40c..69b20f013c 100644
--- a/lcl/controls.pp
+++ b/lcl/controls.pp
@@ -1026,7 +1026,9 @@ type
     {$ENDIF}
     procedure BeginAutoSizing; // set AutoSizing=true, can be used to prevent circles
     procedure EndAutoSizing;   // set AutoSizing=false
+    {$IFDEF OldAutoSize}
     function AutoSizeCanStart: boolean; virtual;
+    {$ENDIF}
     procedure AnchorSideChanged(TheAnchorSide: TAnchorSide); virtual;
     procedure ForeignAnchorSideChanged(TheAnchorSide: TAnchorSide;
                                        Operation: TAnchorSideChangeOperation); virtual;
diff --git a/lcl/forms.pp b/lcl/forms.pp
index da85ca8754..72f29383dd 100644
--- a/lcl/forms.pp
+++ b/lcl/forms.pp
@@ -574,6 +574,9 @@ type
     function IsShortcut(var Message: TLMKey): boolean; virtual;
     procedure MakeFullyVisible(AMonitor: TMonitor = nil; UseWorkarea: Boolean = False);
     function AutoSizeCheckParent: Boolean; override;
+    procedure GetPreferredSize(var PreferredWidth, PreferredHeight: integer;
+                               Raw: boolean = false;
+                               WithThemeSpace: boolean = true); override;
     procedure Release;
     function CanFocus: Boolean; override;
     procedure SetFocus; override;
diff --git a/lcl/include/control.inc b/lcl/include/control.inc
index c2e8caf559..ec65d10372 100644
--- a/lcl/include/control.inc
+++ b/lcl/include/control.inc
@@ -2399,8 +2399,8 @@ procedure TControl.DoAllAutoSize;
     i: Integer;
   begin
     if AControl.AutoSizeDelayed then exit;
-
     if not (cfAutoSizeNeeded in AControl.FControlFlags) then exit;
+
     //DebugLn(['TControl.DoAllAutoSize.AutoSizeControl ',DbgSName(AControl),' AutoSize=',AControl.AutoSize]);
     Exclude(AControl.FControlFlags,cfAutoSizeNeeded);
     if AControl.AutoSize then
@@ -2472,6 +2472,7 @@ begin
   end;
 end;
 
+{$IFDEF OldAutoSize}
 {------------------------------------------------------------------------------
   function TControl.AutoSizeCanStart: boolean;
   
@@ -2505,6 +2506,7 @@ begin
   end;
   {$ENDIF}
 end;
+{$ENDIF}
 
 {$IFNDEF OldAutoSize}
 function TControl.AutoSizePhase: TControlAutoSizePhase;
diff --git a/lcl/include/customform.inc b/lcl/include/customform.inc
index b83a8d4008..eaa853402a 100644
--- a/lcl/include/customform.inc
+++ b/lcl/include/customform.inc
@@ -560,8 +560,6 @@ end;
 procedure TCustomForm.WMPaint(var Message: TLMPaint);
 begin
   //DebugLn('[TCustomForm.WMPaint] ',Name,':',ClassName);
-  if AutoSizingLockCount = 0 then
-    Include(FFormState, fsDisableAutoSize);
   inherited WMPaint(Message);
   //DebugLn('[TCustomForm.WMPaint] END ',Name,':',ClassName);
 end;
@@ -581,6 +579,18 @@ begin
   {$IFDEF CHECK_POSITION}
   DebugLn('[TCustomForm.WMSize] ',DbgSName(Self),' Message.SizeType=',dbgs(Message.SizeType),' Message.Width=',dbgs(Message.Width),' Message.Height=',dbgs(Message.Height));
   {$ENDIF}
+
+  if (Parent=nil) and ((Message.SizeType and SIZE_SourceIsInterface)>0) then
+  begin
+    // this is a top level form (constraints depend on window manager)
+    // and the widgetset set a size
+    if (Message.Width<>Width) or (Message.Height<>Height) then
+    begin
+      // the window manager sets another size => disable autosize to prevent endless loop
+      Include(FFormState,fsDisableAutoSize);
+    end;
+  end;
+
   Assert(False, 'Trace:WMSIZE in TCustomForm');
   if not (csDesigning in ComponentState) then
   begin
@@ -1158,11 +1168,22 @@ begin
   end;
 end;
 
+procedure TCustomForm.GetPreferredSize(var PreferredWidth,
+  PreferredHeight: integer; Raw: boolean; WithThemeSpace: boolean);
+begin
+  if fsDisableAutoSize in FFormState then begin
+    PreferredWidth:=Width;
+    PreferredHeight:=Height;
+  end else begin
+    inherited GetPreferredSize(PreferredWidth, PreferredHeight, Raw,
+      WithThemeSpace);
+  end;
+end;
+
 procedure TCustomForm.DoAutoSize;
 begin
   //DebugLn(['TCustomForm.DoAutoSize ',DbgSName(Self),' ',WindowState=wsNormal,' ',fsDisableAutoSize in FFormState,' ',dbgs(BoundsRect),' ',dbgs(ClientRect)]);
-  if (WindowState=wsNormal) and (not (fsDisableAutoSize in FFormState)) then
-    inherited DoAutoSize;
+  inherited DoAutoSize;
 end;
 
 procedure TCustomForm.SetAutoSize(Value: Boolean);
diff --git a/lcl/include/wincontrol.inc b/lcl/include/wincontrol.inc
index 74f191eb5d..240ee7210f 100644
--- a/lcl/include/wincontrol.inc
+++ b/lcl/include/wincontrol.inc
@@ -3319,7 +3319,7 @@ begin
   {$IFDEF VerboseAllAutoSize}
   debugln('TWinControl.DoAutoSize ',DbgSName(Self));
   {$ENDIF}
-  if (not AutoSizeCanStart) or (AutoSizePhase<>caspComputingBounds) then begin
+  if (AutoSizePhase<>caspComputingBounds) then begin
     {$IFDEF VerboseAllAutoSize}
     DebugLn(['TWinControl.DoAutoSize DELAYED AutoSizeCanStart=',AutoSizeCanStart,' AutoSizeDelayed=',AutoSizeDelayed,' AutoSizePhase=',AutoSizePhaseNames[AutoSizePhase]]);
     {$ENDIF}