Use a direct rendering approach for GTK screenshots.

gtk/src/toga_gtk/window.py

@@ -1,10 +1,10 @@
-import os
+from io import BytesIO
 
 from toga.command import GROUP_BREAK, SECTION_BREAK
 from toga.handlers import wrapped_handler
 
 from .container import TogaContainer
-from .libs import Gdk, Gtk
+from .libs import Gtk, cairo
 
 
 class Window:
@@ -154,36 +154,23 @@ def set_full_screen(self, is_full_screen):
             self.native.unfullscreen()
 
     def get_image_data(self):
-        if os.environ.get("XDG_SESSION_TYPE", "").lower() == "x11":
-            display = self.native.get_display()
-            display.flush()
-
-            # For some reason, converting the *window* to a pixbuf fails. But if
-            # you extract a *part* of the overall screen, that works. So - work
-            # out the origin of the window, then the allocation for the
-            # container relative to that window, and capture that rectangle.
-            window = self.native.get_window()
-            origin = window.get_origin()
-            allocation = self.container.get_allocation()
-
-            screen = display.get_default_screen()
-            root_window = screen.get_root_window()
-            screenshot = Gdk.pixbuf_get_from_window(
-                root_window,
-                origin.x + allocation.x,
-                origin.y + allocation.y,
-                allocation.width,
-                allocation.height,
-            )
-
-            success, buffer = screenshot.save_to_bufferv("png")
-            if success:
-                return buffer
-            else:  # pragma: nocover
-                # This shouldn't ever happen, and it's difficult to manufacture
-                # in test conditions
-                raise ValueError(f"Unable to generate screenshot of {self}")
-        else:  # pragma: no-cover
-            # Wayland requires a different API. We also don't (can't?) run CI
-            # under Wayland, so mark this branch no-cover.
-            raise NotImplementedError("Cannot take screenshots under Wayland.")
+        display = self.native.get_display()
+        display.flush()
+
+        width = self.container.get_allocation().width
+        height = self.container.get_allocation().height
+
+        surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, width, height)
+        context = cairo.Context(surface)
+
+        # Render the background of the window into the Cairo context
+        Gtk.render_background(
+            self.native.get_style_context(), context, 0, 0, width, height
+        )
+
+        # Render the content of the window.
+        self.container.draw(context)
+
+        data = BytesIO()
+        surface.write_to_png(data)
+        return data.getbuffer()