diff --git a/docs/examples/introduction/timers.css b/docs/examples/introduction/timers.css
index 5d6272b7e..ff92cc771 100644
--- a/docs/examples/introduction/timers.css
+++ b/docs/examples/introduction/timers.css
@@ -5,7 +5,7 @@ TimerWidget {
     border: tall $panel-darken-2;
     margin: 1;
     padding: 0 1;
-    transition: background 200ms linear;
+    transition: background 300ms linear;
 }
 
 TimerWidget.started {   
diff --git a/docs/examples/introduction/timers.py b/docs/examples/introduction/timers.py
index fc1abe5a0..d990631ef 100644
--- a/docs/examples/introduction/timers.py
+++ b/docs/examples/introduction/timers.py
@@ -12,6 +12,7 @@ class TimeDisplay(Static):
     time_delta = Reactive(0.0)
 
     def watch_time_delta(self, time_delta: float) -> None:
+        """Called when time_delta changes."""
         minutes, seconds = divmod(time_delta, 60)
         hours, minutes = divmod(minutes, 60)
         self.update(f"{hours:02.0f}:{minutes:02.0f}:{seconds:05.2f}")
@@ -25,20 +26,24 @@ class TimerWidget(Static):
     started = Reactive(False)
 
     def on_mount(self) -> None:
+        """Called when widget is first added."""
         self.update_timer = self.set_interval(1 / 30, self.update_elapsed, pause=True)
 
     def update_elapsed(self) -> None:
+        """Updates elapsed time."""
         self.query_one(TimeDisplay).time_delta = (
             self.total + monotonic() - self.start_time if self.started else self.total
         )
 
     def compose(self) -> ComposeResult:
+        """Composes the timer widget."""
         yield Button("Start", id="start", variant="success")
         yield Button("Stop", id="stop", variant="error")
         yield TimeDisplay()
         yield Button("Reset", id="reset")
 
     def watch_started(self, started: bool) -> None:
+        """Called when the 'started' attribute changes."""
         if started:
             self.start_time = monotonic()
             self.update_timer.resume()
@@ -51,6 +56,7 @@ class TimerWidget(Static):
             self.query_one("#start").focus()
 
     def on_button_pressed(self, event: Button.Pressed) -> None:
+        """Called when a button is pressed."""
         button_id = event.button.id
         self.started = button_id == "start"
         if button_id == "reset":
@@ -62,20 +68,24 @@ class TimerApp(App):
     """Manage the timers."""
 
     def on_load(self) -> None:
+        """Called when the app first loads."""
         self.bind("a", "add_timer", description="Add")
         self.bind("r", "remove_timer", description="Remove")
 
     def compose(self) -> ComposeResult:
+        """Called to ad widgets to the app."""
         yield Header()
         yield Footer()
         yield Container(TimerWidget(), TimerWidget(), TimerWidget(), id="timers")
 
     def action_add_timer(self) -> None:
+        """An action to add a timer."""
         new_timer = TimerWidget()
         self.query_one("#timers").mount(new_timer)
         new_timer.scroll_visible()
 
     def action_remove_timer(self) -> None:
+        """Called to remove a timer."""
         timers = self.query("#timers TimerWidget")
         if timers:
             timers.last().remove()
diff --git a/src/textual/dom.py b/src/textual/dom.py
index 19e9d00f2..630c50aee 100644
--- a/src/textual/dom.py
+++ b/src/textual/dom.py
@@ -1,7 +1,16 @@
 from __future__ import annotations
 
 from inspect import getfile
-from typing import ClassVar, Iterable, Iterator, Type, overload, TypeVar, TYPE_CHECKING
+from typing import (
+    cast,
+    ClassVar,
+    Iterable,
+    Iterator,
+    Type,
+    overload,
+    TypeVar,
+    TYPE_CHECKING,
+)
 
 import rich.repr
 from rich.highlighter import ReprHighlighter
@@ -29,6 +38,14 @@ if TYPE_CHECKING:
     from .widget import Widget
 
 
+class DOMError(Exception):
+    pass
+
+
+class NoScreen(DOMError):
+    pass
+
+
 class NoParent(Exception):
     pass
 
@@ -197,7 +214,8 @@ class DOMNode(MessagePump):
         Returns:
             DOMNode | None: The node which is the direct parent of this node.
         """
-        return self._parent
+
+        return cast("DOMNode | None", self._parent)
 
     @property
     def screen(self) -> "Screen":
@@ -209,7 +227,8 @@ class DOMNode(MessagePump):
         node = self
         while node and not isinstance(node, Screen):
             node = node._parent
-        assert isinstance(node, Screen)
+        if not isinstance(node, Screen):
+            raise NoScreen("{self} has no screen")
         return node
 
     @property
diff --git a/src/textual/reactive.py b/src/textual/reactive.py
index a5d52e70a..f16dc78c3 100644
--- a/src/textual/reactive.py
+++ b/src/textual/reactive.py
@@ -73,7 +73,7 @@ class Reactive(Generic[ReactiveType]):
         if callable(validate_function):
             value = validate_function(value)
         if current_value != value or first_set:
-            setattr(obj, f"{self.internal_name}__first_set", True)
+            setattr(obj, f"{self.internal_name}__first_set", False)
             setattr(obj, self.internal_name, value)
             self.check_watchers(obj, name, current_value)
             if self.layout or self.repaint:
diff --git a/src/textual/widget.py b/src/textual/widget.py
index 9d4a2938a..73cc137c6 100644
--- a/src/textual/widget.py
+++ b/src/textual/widget.py
@@ -37,7 +37,7 @@ from .geometry import Offset, Region, Size, Spacing, clamp
 from .layouts.vertical import VerticalLayout
 from .message import Message
 from .reactive import Reactive
-from ._timer import Timer
+from .dom import NoScreen
 
 
 if TYPE_CHECKING:
@@ -516,6 +516,8 @@ class Widget(DOMNode):
         """The region occupied by this widget, relative to the Screen."""
         try:
             return self.screen.find_widget(self).region
+        except NoScreen:
+            return Region()
         except errors.NoWidget:
             return Region()
 
@@ -526,6 +528,8 @@ class Widget(DOMNode):
         """
         try:
             return self.screen.find_widget(self).virtual_region
+        except NoScreen:
+            return Region()
         except errors.NoWidget:
             return Region()