Wrong side information in WM_SIZING for window with a very small height

delphi, winapi, window

Solution

Looks like a bug to me with the OS. Under the conditions of your test case, hit test handling is wrong, default window procedure returns `HTTOP` when it should return `HTBOTTOM`. You can override hit test handling for a workaround:

procedure TForm1.WMNCHitTest(var Message: TWMNCHitTest);
begin
  inherited;
  if (Message.Result = HTTOP) and
      (Message.Pos.Y > Top + Height - GetSystemMetrics(SM_CYSIZEFRAME)) then
    Message.Result := HTBOTTOM;
end;

Problem

- I create a captionless window. - I resize it manually (or programmatically) so that its height is 30 pixels or less. - When I then grab the bottom border to resize it vertically, it behaves as if I were dragging the top border. Indeed when debugging the program, the WM_SIZING parameter contains WMSZ_TOP instead of WMSZ_BOTTOM. My program is written in Delphi, basically the problem is reproducible with a main form with the following FormCreate: ``` procedure TForm2.FormCreate(Sender: TObject); var oldStyle : LongInt; var newStyle : LongInt; begin // Adapt windows style. oldStyle := WINDOWS.GetWindowLong ( handle, GWL_STYLE); newStyle := oldStyle and (not WS_CAPTION) and (not WS_MAXIMIZEBOX); WINDOWS.SetWindowLong( handle, GWL_STYLE, newStyle); // SetWindowPos with SWP_FRAMECHANGED needs to be called at that point // in order for the style change to be taken immediately into account. WINDOWS.SetWindowPos( handle, 0, 0, 0, 0, 0, SWP_NOZORDER or SWP_NOMOVE or SWP_NOSIZE or SWP_FRAMECHANGED or SWP_NOACTIVATE); end; ```

Original source