diff --git a/CHANGELOG.md b/CHANGELOG.md
index 299c3db889..2531e0c14f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,10 +11,12 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Eager tasks are now enabled On Python3.12 and above https://github.com/Textualize/textual/pull/6102
 
-
 ### Added
 
 - Added `DOMNode.displayed_and_visible_children` https://github.com/Textualize/textual/pull/6102
+- Added `Widget.process_layout` https://github.com/Textualize/textual/pull/6105
+- Added `App.viewport_size` https://github.com/Textualize/textual/pull/6105
+- Added `Screen.size` https://github.com/Textualize/textual/pull/6105
 
 ## [6.1.0] - 2025-08-01
 
diff --git a/src/textual/_arrange.py b/src/textual/_arrange.py
index ddab9b185c..6cdca4af3d 100644
--- a/src/textual/_arrange.py
+++ b/src/textual/_arrange.py
@@ -31,6 +31,11 @@ def _build_layers(widgets: Iterable[Widget]) -> Mapping[str, Sequence[Widget]]:
     return layers
 
 
+_get_dock = attrgetter("styles.is_docked")
+_get_split = attrgetter("styles.is_split")
+_get_display = attrgetter("display")
+
+
 def arrange(
     widget: Widget,
     children: Sequence[Widget],
@@ -50,22 +55,17 @@ def arrange(
     """
 
     placements: list[WidgetPlacement] = []
-    scroll_spacing = Spacing()
-
-    get_dock = attrgetter("styles.is_docked")
-    get_split = attrgetter("styles.is_split")
-    get_display = attrgetter("styles.display")
-
+    scroll_spacing = NULL_SPACING
     styles = widget.styles
 
     # Widgets which will be displayed
-    display_widgets = [child for child in children if get_display(child) != "none"]
+    display_widgets = list(filter(_get_display, children))
     # Widgets organized into layers
     layers = _build_layers(display_widgets)
 
     for widgets in layers.values():
         # Partition widgets into split widgets and non-split widgets
-        non_split_widgets, split_widgets = partition(get_split, widgets)
+        non_split_widgets, split_widgets = partition(_get_split, widgets)
         if split_widgets:
             _split_placements, dock_region = _arrange_split_widgets(
                 split_widgets, size, viewport
@@ -78,7 +78,7 @@ def arrange(
 
         # Partition widgets into "layout" widgets (those that appears in the normal 'flow' of the
         # document), and "dock" widgets which are positioned relative to an edge
-        layout_widgets, dock_widgets = partition(get_dock, non_split_widgets)
+        layout_widgets, dock_widgets = partition(_get_dock, non_split_widgets)
 
         # Arrange docked widgets
         if dock_widgets:
@@ -94,8 +94,10 @@ def arrange(
 
         if layout_widgets:
             # Arrange layout widgets (i.e. not docked)
-            layout_placements = widget.layout.arrange(
-                widget, layout_widgets, dock_region.size, greedy=not optimal
+            layout_placements = widget.process_layout(
+                widget.layout.arrange(
+                    widget, layout_widgets, dock_region.size, greedy=not optimal
+                )
             )
             scroll_spacing = scroll_spacing.grow_maximum(dock_spacing)
             placement_offset = dock_region.offset
diff --git a/src/textual/_profile.py b/src/textual/_profile.py
index 3e880cf15c..8324db9dee 100644
--- a/src/textual/_profile.py
+++ b/src/textual/_profile.py
@@ -10,10 +10,17 @@
 
 
 @contextlib.contextmanager
-def timer(subject: str = "time") -> Generator[None, None, None]:
-    """print the elapsed time. (only used in debugging)"""
+def timer(subject: str = "time", threshold: float = 0) -> Generator[None, None, None]:
+    """print the elapsed time. (only used in debugging).
+
+    Args:
+        subject: Text shown in log.
+        threshold: Time in second after which the log is written.
+
+    """
     start = perf_counter()
     yield
     elapsed = perf_counter() - start
-    elapsed_ms = elapsed * 1000
-    log(f"{subject} elapsed {elapsed_ms:.4f}ms")
+    if elapsed >= threshold:
+        elapsed_ms = elapsed * 1000
+        log(f"{subject} elapsed {elapsed_ms:.4f}ms")
diff --git a/src/textual/app.py b/src/textual/app.py
index 26c3f818f4..92748d0e7d 100644
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -1565,6 +1565,14 @@ def size(self) -> Size:
             width, height = self.console.size
         return Size(width, height)
 
+    @property
+    def viewport_size(self) -> Size:
+        """Get the viewport size (size of the screen)."""
+        try:
+            return self.screen.size
+        except (ScreenStackError, NoScreen):
+            return self.size
+
     def _get_inline_height(self) -> int:
         """Get the inline height (height when in inline mode).
 
diff --git a/src/textual/layouts/grid.py b/src/textual/layouts/grid.py
index 5aa419b057..c6f518360b 100644
--- a/src/textual/layouts/grid.py
+++ b/src/textual/layouts/grid.py
@@ -60,7 +60,7 @@ def arrange(
                     table_size_columns -= 1
 
         table_size_rows = styles.grid_size_rows
-        viewport = parent.screen.size
+        viewport = parent.app.viewport_size
         keyline_style, _keyline_color = styles.keyline
         offset = (0, 0)
         gutter_spacing: Spacing | None
diff --git a/src/textual/layouts/horizontal.py b/src/textual/layouts/horizontal.py
index c1720173d6..fdead10c1f 100644
--- a/src/textual/layouts/horizontal.py
+++ b/src/textual/layouts/horizontal.py
@@ -25,7 +25,7 @@ def arrange(
         parent.pre_layout(self)
         placements: list[WidgetPlacement] = []
         add_placement = placements.append
-        viewport = parent.app.size
+        viewport = parent.app.viewport_size
 
         child_styles = [child.styles for child in children]
         box_margins: list[Spacing] = [
diff --git a/src/textual/layouts/stream.py b/src/textual/layouts/stream.py
index ffae3b58ce..9665f408f6 100644
--- a/src/textual/layouts/stream.py
+++ b/src/textual/layouts/stream.py
@@ -37,7 +37,7 @@ def arrange(
         parent.pre_layout(self)
         if not children:
             return []
-        viewport = parent.app.size
+        viewport = parent.app.viewport_size
 
         _Region = Region
         _WidgetPlacement = WidgetPlacement
@@ -81,3 +81,37 @@ def arrange(
             y += height
 
         return placements
+
+    def get_content_width(self, widget: Widget, container: Size, viewport: Size) -> int:
+        """Get the optimal content width by arranging children.
+
+        Args:
+            widget: The container widget.
+            container: The container size.
+            viewport: The viewport size.
+
+        Returns:
+            Width of the content.
+        """
+        return widget.scrollable_content_region.width
+
+    def get_content_height(
+        self, widget: Widget, container: Size, viewport: Size, width: int
+    ) -> int:
+        """Get the content height.
+
+        Args:
+            widget: The container widget.
+            container: The container size.
+            viewport: The viewport.
+            width: The content width.
+
+        Returns:
+            Content height (in lines).
+        """
+        if widget._nodes:
+            arrangement = widget._arrange(Size(width, 0))
+            height = arrangement.total_region.height
+        else:
+            height = 0
+        return height
diff --git a/src/textual/layouts/vertical.py b/src/textual/layouts/vertical.py
index b229fa6622..4f68449417 100644
--- a/src/textual/layouts/vertical.py
+++ b/src/textual/layouts/vertical.py
@@ -23,7 +23,7 @@ def arrange(
         parent.pre_layout(self)
         placements: list[WidgetPlacement] = []
         add_placement = placements.append
-        viewport = parent.app.size
+        viewport = parent.app.viewport_size
 
         child_styles = [child.styles for child in children]
         box_margins: list[Spacing] = [
diff --git a/src/textual/screen.py b/src/textual/screen.py
index 14e11046da..f721a3913f 100644
--- a/src/textual/screen.py
+++ b/src/textual/screen.py
@@ -341,6 +341,11 @@ def layers(self) -> tuple[str, ...]:
             extras.append("_tooltips")
         return (*super().layers, *extras)
 
+    @property
+    def size(self) -> Size:
+        """The size of the screen."""
+        return self.app.size - self.styles.gutter.totals
+
     def _watch_focused(self):
         self.refresh_bindings()
 
@@ -512,7 +517,7 @@ def get_maximize_widgets(maximized: Widget) -> list[Widget]:
                 else self._nodes
             ),
             size,
-            self.screen.size,
+            self.size,
             False,
         )
 
diff --git a/src/textual/widget.py b/src/textual/widget.py
index 63ab9664c1..ae298fe121 100644
--- a/src/textual/widget.py
+++ b/src/textual/widget.py
@@ -79,7 +79,7 @@
     Spacing,
     clamp,
 )
-from textual.layout import Layout
+from textual.layout import Layout, WidgetPlacement
 from textual.layouts.vertical import VerticalLayout
 from textual.message import Message
 from textual.messages import CallbackType, Prune
@@ -460,7 +460,7 @@ def __init__(
         self._content_height_cache: tuple[object, int] = (None, 0)
 
         self._arrangement_cache: FIFOCache[
-            tuple[Size, int, Widget], DockArrangeResult
+            tuple[Size, int, Widget | None], DockArrangeResult
         ] = FIFOCache(4)
 
         self._styles_cache = StylesCache()
@@ -605,7 +605,7 @@ def offset(self) -> Offset:
         Returns:
             Relative offset.
         """
-        return self.styles.offset.resolve(self.size, self.app.size)
+        return self.styles.offset.resolve(self.size, self.screen.size)
 
     @offset.setter
     def offset(self, offset: tuple[int, int]) -> None:
@@ -717,10 +717,26 @@ def _cover(self, widget: Widget) -> None:
         self._uncover()
         self._cover_widget = widget
         widget._parent = self
+        widget._start_messages()
         widget._post_register(self.app)
         self.app.stylesheet.apply(widget)
         self.refresh(layout=True)
-        widget._start_messages()
+
+    def process_layout(
+        self, placements: list[WidgetPlacement]
+    ) -> list[WidgetPlacement]:
+        """A hook to allow for the manipulation of widget placements before rendering.
+
+        You could use this as a way to modify the positions / margins of widgets if your requirement is
+        not supported in TCSS. In practice, this method is rarely needed!
+
+        Args:
+            placements: A list of [`WidgetPlacement`][textual.layout.WidgetPlacement] objects.
+
+        Returns:
+            A new list of placements.
+        """
+        return placements
 
     def _uncover(self) -> None:
         """Remove any widget, previously set via [`_cover`][textual.widget.Widget._cover]."""