The use of the resize-grip-visible property of GtkWindow will be deprecated in the upcoming gtk+ release, unless application developers provide arguments for its use (as per commit message https://git.gnome.org/browse/gtk+/commit/?id=ece9ae4bc507a2f2e0d6660a0058c0f79735736b). Use of property in webkit gtk: https://github.com/WebKit/webkit/blob/d5b43f2ad6bb4e3e073d0dac88f27cee7da831ee/Source/WebKit2/UIProcess/API/gtk/WebKitWebViewBase.cpp#L151
This doesn't seem to be in use in WebKit for a long time now. *** This bug has been marked as a duplicate of bug 135699 ***