Compositor ignores non-mounted widgets.

This, in turn, ensures widgets are not rendered before they are mounted.

CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Off-by-one in CSS error reporting https://github.com/Textualize/textual/issues/3625
 - Loading indicators and app notifications overlapped in the wrong order https://github.com/Textualize/textual/issues/3677
 - Widgets being loaded are disabled and have their scrolling explicitly disabled too https://github.com/Textualize/textual/issues/3677
+- Method render on a widget could be called before mounting said widget https://github.com/Textualize/textual/issues/2914
 
 ### Added
 

src/textual/_compositor.py

@@ -573,6 +573,9 @@ class Compositor:
                 visible: Whether the widget should be visible by default.
                     This may be overridden by the CSS rule `visibility`.
             """
+            if not widget._is_mounted:
+                return
+
             styles = widget.styles
             visibility = styles.get_rule("visibility")
             if visibility is not None:

src/textual/app.py

@@ -2202,6 +2202,7 @@ class App(Generic[ReturnType], DOMNode):
                         self.check_idle()
                     finally:
                         self._mounted_event.set()
+                        self._is_mounted = True
 
                     Reactive._initialize_object(self)
 

src/textual/message_pump.py

@@ -122,6 +122,13 @@ class MessagePump(metaclass=_MessagePumpMeta):
         self._last_idle: float = time()
         self._max_idle: float | None = None
         self._mounted_event = asyncio.Event()
+        self._is_mounted = False
+        """Having this explicit Boolean is an optimization.
+
+        The same information could be retrieved from `self._mounted_event.is_set()`, but
+        we need to access this frequently in the compositor and the attribute with the
+        explicit Boolean value is faster than the two lookups and the function call.
+        """
         self._next_callbacks: list[events.Callback] = []
         self._thread_id: int = threading.get_ident()
 
@@ -508,6 +515,7 @@ class MessagePump(metaclass=_MessagePumpMeta):
         finally:
             # This is critical, mount may be waiting
             self._mounted_event.set()
+            self._is_mounted = True
         return True
 
     def _post_mount(self):
@@ -547,6 +555,7 @@ class MessagePump(metaclass=_MessagePumpMeta):
                 raise
             except Exception as error:
                 self._mounted_event.set()
+                self._is_mounted = True
                 self.app._handle_exception(error)
                 break
             finally:

src/textual/widget.py

@@ -396,7 +396,7 @@ class Widget(DOMNode):
     @property
     def is_mounted(self) -> bool:
         """Check if this widget is mounted."""
-        return self._mounted_event.is_set()
+        return self._is_mounted
 
     @property
     def siblings(self) -> list[Widget]:

tests/test_mount.py

@@ -0,0 +1,27 @@
+"""Regression test for https://github.com/Textualize/textual/issues/2914
+
+Make sure that calls to render only happen after a widget being mounted.
+"""
+
+import asyncio
+
+from textual.app import App
+from textual.widget import Widget
+
+
+class W(Widget):
+    def render(self):
+        return self.renderable
+
+    async def on_mount(self):
+        await asyncio.sleep(0.1)
+        self.renderable = "1234"
+
+
+async def test_render_only_after_mount():
+    """Regression test for https://github.com/Textualize/textual/issues/2914"""
+    app = App()
+    async with app.run_test() as pilot:
+        app.mount(W())
+        app.mount(W())
+        await pilot.pause()