Snapshot test to keep tabs on opacity.

This snapshot will be used to make sure that the opacity is being taken into account when rendering widgets. This shows that some of the issues in #3304 and #3413.

tests/snapshot_tests/snapshot_apps/component_classes_opacity.py

@@ -0,0 +1,72 @@
+from textual.app import App, ComposeResult
+from textual.widgets import (
+    Checkbox,
+    OptionList,
+    DataTable,
+    DirectoryTree,
+    Footer,
+    Input,
+    ProgressBar,
+    RadioButton,
+    SelectionList,
+    Switch,
+    Tree,
+)
+from textual.widgets.option_list import Option
+
+
+class ComponentClassesOpacity(App[None]):
+    BINDINGS = [("n", "notification", "random Notification")]
+
+    CSS = """
+    * {
+        max-height: 3;
+    }
+    Checkbox > .toggle--label,
+    DataTable > .datatable--header,
+    DirectoryTree > .directory-tree--file,
+    DirectoryTree > .directory-tree--folder,
+    Footer > .footer--description,
+    Footer > .footer--key,
+    Input > .input--placeholder,
+    OptionList > .option-list--option-highlighted,
+    ProgressBar Bar > .bar--indeterminate,
+    RadioButton > .toggle--label,
+    SelectionList > .selection-list--button-highlighted,
+    Switch > .switch--slider,
+    Toast > .toast--title,
+    Tree > .tree--label {
+        text-opacity: 0%;
+        color: white;
+    }
+    """
+
+    def compose(self) -> ComposeResult:
+        yield Checkbox("this should be invisible")
+        dt = DataTable()
+        dt.add_column("this should be invisible")
+        yield dt
+        yield DirectoryTree(".")
+        yield Footer()
+        yield Input(placeholder="this should be invisible")
+        yield OptionList(Option("this should be invisible"))
+        yield ProgressBar()
+        yield RadioButton("this should be invisible")
+        yield SelectionList(("this should be invisible", 0))
+        yield Switch()
+        tree: Tree = Tree("Dune")
+        tree.root.expand()
+        characters = tree.root.add("Characters", expand=True)
+        characters.add_leaf("Paul")
+        characters.add_leaf("Jessica")
+        characters.add_leaf("Chani")
+        yield tree
+
+    def action_notification(self):
+        self.notify(
+            title="this should be invisible", message="this should be invisible"
+        )
+
+
+if __name__ == "__main__":
+    ComponentClassesOpacity().run()