Tweak visibility handling to ensure complete coverage in testbed cond…

…itions.

gtk/src/toga_gtk/container.py

@@ -178,10 +178,7 @@ def do_size_allocate(self, allocation):
             # the Toga widget. Toga maintains a tree of children; all nodes
             # in that tree are direct children of the container.
             for widget in self.get_children():
-                if not widget.get_visible():
-                    # print("  not visible {widget.interface}")
-                    pass
-                else:
+                if widget.get_visible():
                     # Set the size of the child widget to the computed layout size.
                     # print(f"  allocate child {widget.interface}: {widget.interface.layout}")
                     widget_allocation = Gdk.Rectangle()

gtk/src/toga_gtk/widgets/base.py

@@ -152,6 +152,8 @@ def set_alignment(self, alignment):
 
     def set_hidden(self, hidden):
         self.native.set_visible(not hidden)
+        if self.container:
+            self.container.make_dirty()
 
     def set_color(self, color):
         self.apply_css("color", get_color_css(color))