diff --git a/lcl/include/wincontrol.inc b/lcl/include/wincontrol.inc
index 298bba7c02..37346973b5 100644
--- a/lcl/include/wincontrol.inc
+++ b/lcl/include/wincontrol.inc
@@ -4058,12 +4058,16 @@ begin
           // they will probably change
           // to avoid resizes it is better use the defaults
           GetDefaults(Result);
-        end else begin
+        end else
+        if not (self is TCustomForm) then
+        begin
           // -> adjust client rect based on LCL bounds
           // for example: if the Width in LCL differ from the Width of the Interface
           // object, then adjust the clientwidth accordingly
           // this often anticipates later LM_SIZE messages from the interface
           // and reduces resizes
+          // BUT: Do this only when not a form. Form has its own clientrect specification:
+          // https://wiki.lazarus.freepascal.org/Lazarus_FAQ#Why_are_TForm.ClientWidth.2FClientHeight_the_same_as_TForm.Width.2FHeight
           inc(Result.Right,Width-InterfaceWidth);
           inc(Result.Bottom,Height-InterfaceHeight);
         end;