Improve support of child windows that may leak outside their parent window.

1) add Wayland code that prevent subwindows from leaking outside their parent.
This does not cover GL subwindows.

2) add macOS code that prevent GL subwindows from leaking outside their parent.
This fixes issue #494 for the macOS platform.

N.B.: Wayland GL subwindows are not prevented from leaking because no solution
that would not require any change in client applications was found. Code that
would cover Wayland GL subwindows but would require client applications to always
use the FL_ALPHA flag is included in this commit in commented out form.

1 files changed, 5 insertions, 0 deletions

diff --git a/FL/Fl_Gl_Window.H b/FL/Fl_Gl_Window.H
index 9143d0a30..2bd1dafaf 100644
--- a/FL/Fl_Gl_Window.H
+++ b/FL/Fl_Gl_Window.H
@@ -47,6 +47,11 @@ class Fl_Gl_Window_Driver;
   to add a selection of widgets to an OpenGL window. The widgets will draw on top
   of any OpenGL rendering. The number of supported widgets will increase as the
   driver development improves. Program test/cube.cxx illustrates how to do that.
+ 
+  \note FLTK expects that when an Fl_Gl_Window is a child of a parent Fl_Window,
+  the child window lies entirely inside its parent window. If that's not the case, what
+  happens to the part of the GL subwindow which leaks outside its parent is undefined
+  and susceptible to be platform-specific.
 */
 class FL_EXPORT Fl_Gl_Window : public Fl_Window {
   friend class Fl_Gl_Window_Driver;