DnD detection windows visible on Wayland

[sbstnk]

We got this bug reported against gnome-shell when running on Wayland under VMware about small empty square windows popping up in some situations: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4601

Apparently the windows in that bug are DnD detection windows created by vmware-tools. On Wayland these windows are created as regular windows and would have a background color depending on the theme. Then this code tries to make them invisible using set_opacity() which does nothing on Wayland.

An alternative that should work on Wayland is to use CSS to make the background transparent:

• during the DnD plugin initialization (or somewhere that only gets run once) create and add a global CSS provider:
  • gtk_css_provider_new()
  • gtk_css_provider_load_from_*() with the following content: .dnd-detection-window { background: transparent; }
  • gtk_style_context_add_provider_for_screen(gdk_screen_get_default(), ..., GTK_STYLE_PROVIDER_PRIORITY_APPLICATION)
• then add the style class to the window so it will get applied
  • gtk_widget_get_style_context(...)
  • gtk_style_context_add_class(..., "dnd-detection-window")

This is untested and I'm not a gtk expert, so maybe there are better ways to do this, but it should at least point you in the right direction.

[dsouders]

@sbstnk Thank you for reporting this issue and for identifying the problem! An internal bug has been filed with our graphics team to address the problem.