diff --git a/lcl/interfaces/qt/qtint.pp b/lcl/interfaces/qt/qtint.pp
index d761d012c8..177add6819 100644
--- a/lcl/interfaces/qt/qtint.pp
+++ b/lcl/interfaces/qt/qtint.pp
@@ -260,6 +260,8 @@ const
 
    // needed by itemviews (TQtListWidget, TQtTreeWidget)
    LCLQt_ItemViewAfterMouseRelease = QEventType(Ord(QEventUser) + $1008);
+   // used by TQtTabWidget
+   LCLQt_DelayLayoutRequest = QEventType(Ord(QEventUser) + $1009);
 
 var
   QtWidgetSet: TQtWidgetSet;
diff --git a/lcl/interfaces/qt/qtpagecontrol.inc b/lcl/interfaces/qt/qtpagecontrol.inc
index 35c763fccd..bb70e07441 100644
--- a/lcl/interfaces/qt/qtpagecontrol.inc
+++ b/lcl/interfaces/qt/qtpagecontrol.inc
@@ -137,7 +137,55 @@ begin
     QtTabWidget.AttachEvents;
     Result := TLCLIntfHandle(QtTabWidget);
   end;
+end;
 
+class function TQtWSCustomTabControl.GetDefaultClientRect(
+  const AWinControl: TWinControl; const aLeft, aTop, aWidth, aHeight: integer;
+  var aClientRect: TRect): boolean;
+var
+  dx, dy: integer;
+  ATabWidget: TQtTabWidget;
+begin
+  Result := False;
+  if AWinControl.HandleAllocated then
+  begin
+    if TQtWidget(AWinControl.Handle).ChildOfComplexWidget <> ccwTTabControl then
+    begin
+      ATabWidget := TQtTabWidget(AWinControl.Handle);
+      if ATabWidget.testAttribute(QtWA_PendingResizeEvent) then
+      begin
+        // we must recalculate aclientrect since we have pending resize event
+        // and clientrect won't be accurate.issue #21805
+        Result := True;
+        dx := GetPixelMetric(QStylePM_DefaultFrameWidth, nil, nil);
+
+        if ATabWidget.getTabPosition in [QTabWidgetNorth, QTabWidgetSouth] then
+        begin
+          if ATabWidget.TabBar.getVisible then
+            dy := ATabWidget.TabBar.getHeight
+          else
+            dy := 0;
+          aClientRect := Rect(0, 0,
+               Max(0, aWidth - (dx * 2)),
+               Max(0, aHeight - dx  - dy));
+        end else
+        begin
+          if ATabWidget.TabBar.getVisible then
+            dy := ATabWidget.TabBar.getWidth
+          else
+            dy := 0;
+          aClientRect := Rect(0,0,
+               Max(0, aWidth - dx  - dy),
+               Max(0, aHeight - (dx * 2)));
+        end;
+      end;
+    end;
+  end else
+  begin
+    dx := GetPixelMetric(QStylePM_TabBarBaseHeight, nil, nil);
+    aClientRect := Rect(0,0, Max(0, aWidth - (dx * 2)), Max(0, aHeight - (dx * 2)));
+    Result := True;
+  end;
 end;
 
 class procedure TQtWSCustomTabControl.AddPage(const ATabControl: TCustomTabControl;
diff --git a/lcl/interfaces/qt/qtwidgets.pas b/lcl/interfaces/qt/qtwidgets.pas
index 3e3601c132..9e95a9958e 100644
--- a/lcl/interfaces/qt/qtwidgets.pas
+++ b/lcl/interfaces/qt/qtwidgets.pas
@@ -8723,12 +8723,13 @@ end;
 
 function TQtTabWidget.EventFilter(Sender: QObjectH; Event: QEventH): Boolean;
   cdecl;
-{$IFDEF QT_ENABLE_LCL_PAINT_TABS}
 var
+{$IFDEF QT_ENABLE_LCL_PAINT_TABS}
   R: TRect;
   TabGeom: TRect;
   Pt: TPoint;
 {$ENDIF}
+  ALCLEvent: QLCLMessageEventH;
 begin
 
   Result := False;
@@ -8743,13 +8744,11 @@ begin
       ' LCLObject=', dbgsName(LCLObject),
       ' Event=', EventTypeToStr(Event),' inUpdate=',inUpdate);
     {$endif}
-    case QEvent_type(Event) of
-      QEventResize:
-        if LCLObject.ClientRectNeedsInterfaceUpdate then
-          LCLObject.InvalidateClientRectCache(False);
-      // layoutRequest from stacked widget is very important !
-      QEventLayoutRequest: LCLObject.DoAdjustClientRectChange(False);
-    end;
+    {
+      we don't use QEventResize and QEventLayoutRequest of StackedWidget anymore.
+      see commited revision for issue #21805.
+      We are leaving this part for debugging purposes.
+    }
     exit;
   end;
 
@@ -8771,6 +8770,25 @@ begin
           QEvent_ignore(Event);
       end;
     {$ENDIF}
+    LCLQt_DelayLayoutRequest:
+    begin
+      if LCLObject.ClientRectNeedsInterfaceUpdate then
+      begin
+        {$IFDEF VerboseQtEvents}
+        DebugLn('<*><*> TQtTabWidget calling DoAdjustClientRectChange() from LCLQt_DelayLayoutRequest ...');
+        {$ENDIF}
+        LCLObject.DoAdjustClientRectChange(True);
+      end;
+      Result := True;
+    end;
+    QEventLayoutRequest:
+    begin
+      //behaviour is changed because of issue #21805.
+      //we send delayed event so LCL can read clientRect at the right time.
+      ALCLEvent := QLCLMessageEvent_create(LCLQt_DelayLayoutRequest, 0,
+        0, 0, 0);
+      QCoreApplication_postEvent(Sender, ALCLEvent);
+    end;
     QEventKeyPress,
     QEventKeyRelease: QEvent_ignore(Event);
     QEventWheel:
@@ -8829,7 +8847,7 @@ end;
 
 function TQtTabWidget.getClientBounds: TRect;
 begin
-  QWidget_contentsRect(StackWidget, @Result)
+  QWidget_contentsRect(StackWidget, @Result);
 end;
 
 function TQtTabWidget.getCurrentIndex: Integer;
@@ -15285,10 +15303,21 @@ end;
 
 function TQtPage.EventFilter(Sender: QObjectH; Event: QEventH): Boolean; cdecl;
 begin
-  if (QEvent_type(Event) = QEventResize) and
-    LCLObject.Parent.ClientRectNeedsInterfaceUpdate  then
-      LCLObject.Parent.InvalidateClientRectCache(False);
-  Result:=inherited EventFilter(Sender, Event);
+  if (QEvent_type(Event) = QEventResize) then
+  begin
+    //behaviour is changed because of issue #21805.
+    if LCLObject.ClientRectNeedsInterfaceUpdate then
+    begin
+      {$IFDEF VerboseQtEvents}
+      DebugLn('<*><*> TQtPage resize event invalidating clientrect cache !!!');
+      {$ENDIF}
+      LCLObject.InvalidateClientRectCache(False);
+
+      if FChildOfComplexWidget = ccwTabWidget then
+        exit(False);
+    end;
+  end;
+  Result := inherited EventFilter(Sender, Event);
 end;
 
 function TQtPage.getIcon: QIconH;
diff --git a/lcl/interfaces/qt/qtwscomctrls.pp b/lcl/interfaces/qt/qtwscomctrls.pp
index 44c5929005..affe3cae0e 100644
--- a/lcl/interfaces/qt/qtwscomctrls.pp
+++ b/lcl/interfaces/qt/qtwscomctrls.pp
@@ -57,7 +57,9 @@ type
   published
     class function  CreateHandle(const AWinControl: TWinControl;
           const AParams: TCreateParams): TLCLIntfHandle; override;
-
+    class function GetDefaultClientRect(const AWinControl: TWinControl;
+             const {%H-}aLeft, {%H-}aTop, aWidth, aHeight: integer; var aClientRect: TRect
+             ): boolean; override;
     class procedure AddPage(const ATabControl: TCustomTabControl;
       const AChild: TCustomPage; const AIndex: integer); override;
     class procedure MovePage(const ATabControl: TCustomTabControl;
@@ -262,7 +264,7 @@ type
 
 
 implementation
-uses qtint;
+uses qtint, math;
 
 {$include qtpagecontrol.inc}