Improve focus navigation performance by caching Screen.focus_chain between layout changes

src/textual/dom.py

@@ -492,6 +492,10 @@ def trap_focus(self, trap_focus: bool = True) -> None:
             trap_focus: `True` to trap focus. `False` to restore default behavior.
         """
         self._trap_focus = trap_focus
+        try:
+            self.screen._focus_chain_cache = None
+        except Exception:
+            pass
 
     def run_worker(
         self,
@@ -1328,6 +1332,10 @@ def _add_children(self, *nodes: Widget) -> None:
             node._attach(self)
             _append(node)
             node._add_children(*node._pending_children)
+        try:
+            self.screen._focus_chain_cache = None
+        except Exception:
+            pass
 
     WalkType = TypeVar("WalkType", bound="DOMNode")
 

src/textual/screen.py

@@ -326,6 +326,9 @@ def __init__(
         self._css_update_count = -1
         """Track updates to CSS."""
 
+        self._focus_chain_cache: list[Widget] | None = None
+        """Cached focus chain, invalidated on layout or focus change."""
+
         self._layout_widgets: dict[DOMNode, set[Widget]] = {}
         """Widgets whose layout may have changed."""
 
@@ -771,8 +774,8 @@ def _select_all_in_widget(self, widget: Widget) -> None:
     @property
     def focus_chain(self) -> list[Widget]:
         """A list of widgets that may receive focus, in focus order."""
-        # TODO: Calculating a focus chain is moderately expensive.
-        # Suspect we can move focus without calculating the entire thing again.
+        if self._focus_chain_cache is not None:
+            return self._focus_chain_cache
 
         widgets: list[Widget] = []
         add_widget = widgets.append
@@ -823,6 +826,7 @@ def focus_chain(self) -> list[Widget]:
                 if node_is_visible and node.allow_focus():
                     add_widget(node)
 
+        self._focus_chain_cache = widgets
         return widgets
 
     def _move_focus(
@@ -1113,6 +1117,7 @@ def set_focus(
             # Widget is already focused
             return
 
+        self._focus_chain_cache = None
         focused: Widget | None = None
         blurred: Widget | None = None
 
@@ -1423,6 +1428,7 @@ async def _on_layout(self, message: messages.Layout) -> None:
                 break
             widget = ancestor
 
+        self._focus_chain_cache = None
         if layout_required and not self._layout_required:
             self._layout_required = True
             self.check_idle()