Add a unit test for provider discovery

tests/command_palette/test_discover.py

@@ -0,0 +1,34 @@
+from textual.app import App
+from textual.command import CommandPalette, DiscoveryHit, Hit, Hits, Provider
+from textual.widgets import OptionList
+
+
+class SimpleSource(Provider):
+
+    async def discover(self) -> Hits:
+        def goes_nowhere_does_nothing() -> None:
+            pass
+
+        yield DiscoveryHit("XD-1", goes_nowhere_does_nothing, "XD-1")
+
+    async def search(self, query: str) -> Hits:
+        def goes_nowhere_does_nothing() -> None:
+            pass
+
+        yield Hit(1, query, goes_nowhere_does_nothing, query)
+
+
+class CommandPaletteApp(App[None]):
+    COMMANDS = {SimpleSource}
+
+    def on_mount(self) -> None:
+        self.action_command_palette()
+
+
+async def test_discovery_visible() -> None:
+    """A provider with discovery should cause the command palette to be opened right away."""
+    async with CommandPaletteApp().run_test() as pilot:
+        assert CommandPalette.is_open(pilot.app)
+        results = pilot.app.screen.query_one(OptionList)
+        assert results.visible is True
+        assert results.option_count == 1