Running with Roland's suggestion to use a static text control, and based on the "Resize Response" free code sample he generously makes available to all on his TopWizProgramming.com web site, I have created a resizable response window that contains a "resize grip" drag control in the lower-right corner. With Roland's permission, I've attached the enhanced sample application here. The sample app was written in PB 2017 R2 and has been tested under Windows 7 and 10. The code should be easily adaptable to other types of resizable windows (main/sheet, etc.)

During development, I was also able to create a version that utilizes the DrawFrameControl Windows API. It worked, but it required the use of several additional WinAPI functions and more involved code. As others have noted, the DrawFrameControl API is from the Windows XP era and it has not been modernized. The grip it renders is limited to a background color of "button face" and I'm told it also does not behave well in alternative dots-per-inch (DPI) conditions (it is not "DPI-aware"), so I went with the static text control that uses the Marlett font. The sample app illustrates how the grip appears with some alternative window background colors.

The main key to getting this to work correctly was using WinAPI functions that allows the window to capture the mouse cursor during the "drag/resize" operation, then release it. Another key was that the "grip" static text control must be disabled so that it does not receive focus when you click on it to drag/resize the window. There really is not much code needed to make it work, surprisingly.

As Chris has mentioned in this thread and most of you know, PB Themes obviously significantly alters an application's visual appearance. I've not tested the resize grip with themes, but I would be very surprised if it works...so be aware that it was not intended for use with themes.

Miguel, I hope this is something you can use. Have fun!