Thanks for the patch. I'm not sure though if drawing extra lines and/or a rectangle is the correct solution in general and particularly in this platform agnostic way because the title say that the issue is macOS specific. However, I can believe that we can accept this anyway.

Since I can't test and verify this right now I'm leaving this to others (e.g. @ManoloFLTK ?) to decide.

I'm not keen to fix a macOS-speciic issue with platform-independent code that will also be activated on non macOS platforms.

@ggarra13 wrote:

… the problem is also there on other platforms. You just don’t see it as windows are created blank by default there.

I agree that this might be an issue on other platforms as well (pixels that are never drawn). Is it possible to make a short and complete demo program that would show the issue on all platforms, for instance by setting a background color explicitly? One option to do so is using the command line switch -bg "color", e.g. -bg '#554433' for a dark background.

Another important point to consider: the proposed patch changes box drawing code which is applied all over the FLTK GUI, not only for menus. Backgrounds can have more than just one color, for instance the tiled stipple pattern of the plastic scheme or even a user defined image. In such cases the patch might cause other unwanted artifacts.

That all said, IMO the proposed patch is not a proper solution of this issue. A correct patch would draw the extra background pixels only:

• on the macOS platform (maybe) and
• in the context of menus (menu windows)

Otherwise unwanted artifacts would likely cause another regression.

Disclaimer: I didn't test anything, these are just my thoughts while reading about this issue.

I am closing this pull request as I have a better fix for the menus on macOS. Please refer to the PR menuwindow_macos.