WIP: Starting work on adding before/after mount directions

This is still a work-in-progress, but this feels like a good point to commit
for safe keeping. This is a non-working WIP.

src/textual/app.py

@@ -822,16 +822,29 @@ class App(Generic[ReturnType], DOMNode):
         self._require_stylesheet_update.add(self.screen if node is None else node)
         self.check_idle()
 
-    def mount(self, *widgets: Widget) -> AwaitMount:
-        """Mount the given widgets.
+    def mount(
+        self, *widgets: Widget, before: MountSpot = None, after: MountSpot = None
+    ) -> AwaitMount:
+        """Mount the given widgets relative to the app's screen.
 
         Args:
             *widgets (Widget): The widget(s) to mount.
+            before (MountSpot, optional): Optional location to mount before.
+            after (MountSpot, optional): Optional location to mount after.
 
         Returns:
             AwaitMount: An awaitable object that waits for widgets to be mounted.
+
+        Raises:
+            MountError: If there is a problem with the mount request.
+
+        Note:
+            Only one of ``before`` or ``after`` can be provided. If both are
+            provided a ``MountError`` will be raised.
         """
-        return AwaitMount(self._register(self.screen, *widgets))
+        return AwaitMount(
+            self._register(self.screen, *widgets, before=before, after=after)
+        )
 
     def mount_all(self, widgets: Iterable[Widget]) -> AwaitMount:
         """Mount widgets from an iterable.
@@ -1300,12 +1313,20 @@ class App(Generic[ReturnType], DOMNode):
             return True
         return False
 
-    def _register(self, parent: DOMNode, *widgets: Widget) -> list[Widget]:
+    def _register(
+        self,
+        parent: DOMNode,
+        *widgets: Widget,
+        before: MountSpot = None,
+        after: MountSpot = None,
+    ) -> list[Widget]:
         """Register widget(s) so they may receive events.
 
         Args:
             parent (DOMNode): Parent node.
             *widgets: The widget(s) to register.
+            before (MountSpot, optional): Optional location to mount before.
+            after (MountSpot, optional): Optional location to mount after.
 
         Returns:
             list[Widget]: List of modified widgets.

src/textual/dom.py

@@ -13,6 +13,7 @@ from typing import (
     TypeVar,
     cast,
     overload,
+    Union,
 )
 
 import rich.repr
@@ -894,3 +895,56 @@ class DOMNode(MessagePump):
 
     def refresh(self, *, repaint: bool = True, layout: bool = False) -> None:
         pass
+
+    DOMSpot = Union[int, str, "DOMQuery[Widget]", "Widget", None]
+    """The type of a relative location of a node in the DOM."""
+
+    def _find_spot(self, spot: DOMSpot) -> tuple["DOMNode", int]:
+        """Collapse a number of DOM location identifiers into a parent/child-index pair.
+
+        Args:
+            spot (DOMSpot): The spot to find.
+
+        Returns:
+            tuple[DOMNode, int]: The parent and the location in its child list.
+
+        Raises:
+            ValueError: If a given node can't be located amongst children.
+
+        The rules of this method are:
+
+        - Given ``None``, parent is ``self`` and location is ``-1``.
+        - Given an integer, parent is ``self`` and location is the integer value.
+        - Given a DOMNode, parent is the node's parent and location is
+          where the widget is found in the parent's ``children``. If it
+          can't be found a ``ValueError`` will be raised.
+        - Given a query result, the ``first`` node is used. The code then
+          falls to acting as if a DOMNode were given.
+        - Given a string, it is used to perform a query and then the result
+          is used as if a query result were given.
+        """
+
+        from .widget import Widget
+
+        # None pretty much means "at the end of our child list."
+        if spot is None:
+            return cast(Widget, self), -1
+
+        # A numeric location means at that point in our child list.
+        if isinstance(spot, int):
+            return cast(Widget, self), spot
+
+        # We've got a widget that has a parent; let's look for that in our children.
+        if isinstance(spot, DOMNode):
+            try:
+                return cast(DOMNode, spot.parent), spot.parent.children._index(spot)
+            except ValueError:
+                raise DOMError(f"{spot!r} is not a child of {self!r}") from None
+
+        # Do we have a string? If we do, cast that into a query.
+        if isinstance(spot, str):
+            spot = self.query(spot)
+
+        # At this point, we should have a query of some description. So
+        # let's now descend into it and see.
+        return spot.first(DOMNode).parent._find_spot(spot.first(DOMNode))

src/textual/widget.py

@@ -51,7 +51,7 @@ from .reactive import Reactive
 from .render import measure
 
 if TYPE_CHECKING:
-    from .app import App, ComposeResult
+    from .app import App, ComposeResult, MountSpot
     from .scrollbar import (
         ScrollBar,
         ScrollBarCorner,
@@ -375,16 +375,29 @@ class Widget(DOMNode):
         if self._scrollbar_corner is not None:
             yield self._scrollbar_corner
 
-    def mount(self, *widgets: Widget) -> AwaitMount:
-        """Mount child widgets (making this widget a container).
+    def mount(
+        self, *widgets: Widget, before: MountSpot = None, after: MountSpot = None
+    ) -> AwaitMount:
+        """Mount widgets below this widget (making this widget a container).
 
         Args:
             *widgets (Widget): The widget(s) to mount.
+            before (MountSpot, optional): Optional location to mount before.
+            after (MountSpot, optional): Optional location to mount after.
 
         Returns:
             AwaitMount: An awaitable object that waits for widgets to be mounted.
+
+        Raises:
+            MountError: If there is a problem with the mount request.
+
+        Note:
+            Only one of ``before`` or ``after`` can be provided. If both are
+            provided a ``MountError`` will be raised.
         """
-        return AwaitMount(self.app._register(self, *widgets))
+        return AwaitMount(
+            self.app._register(self, *widgets, before=before, after=after)
+        )
 
     def compose(self) -> ComposeResult:
         """Called by Textual to create child widgets.

tests/test_dom_spot.py

@@ -0,0 +1,22 @@
+from textual.widget import Widget
+
+class Content(Widget):
+    pass
+
+class Body(Widget):
+    pass
+
+def test_find_dom_spot():
+    screen = Widget(name="Screen")
+    header = Widget(name="Header", id="header")
+    body   = Body(id="body")
+    content = [ Content(id=f"item{n}") for n in range(1000)]
+    body._add_children(*content)
+    footer = Widget(name="Footer", id="footer")
+    screen._add_children(header, body, footer)
+    assert list(screen.children) == [header,body,footer]
+    assert screen._find_spot(None) == (screen,-1)
+    assert screen._find_spot(1) == (screen, 1)
+    assert screen._find_spot(body) == screen._find_spot(1)
+    assert screen._find_spot("Body") == screen._find_spot(body)
+    assert screen._find_spot("#body") == screen._find_spot(1)