Allow textual widgets to defer mounting children

- Textual does not permit mounting widgets on to other widgets that are
  not themselves mounted.
- This implements a callback mechanism for widgets to mount their
  children they are mounted.

Author: rmartin16

changes/2822.bugfix.rst

@@ -0,0 +1 @@
+The Textual backend is now compatible with versions of Textual after v0.63.3.

textual/src/toga_textual/widgets/base.py

@@ -1,5 +1,9 @@
+from collections import OrderedDict
+from collections.abc import Callable
+
 from travertino.size import at_least
 
+from textual.widget import Widget as TextualWidget
 from toga.style.pack import ROW
 from toga.types import Size
 
@@ -74,7 +78,7 @@ def set_bounds(self, x, y, width, height):
 
         # Positions are more complicated. The (x,y) provided as an argument is
         # in absolute coordinates. The `content_left` ad `content_right` values
-        # of the layout are relative coordianate. Textual doesn't allow specifying
+        # of the layout are relative coordinate. Textual doesn't allow specifying
         # either absolute *or* relative - we can only specify margin values within
         # a row/column box. This means we need to reverse engineer the margins from
         # the computed layout.
@@ -158,7 +162,7 @@ def insert_child(self, index, child):
         pass
 
     def remove_child(self, child):
-        self.native.remove(child.native)
+        self.native.remove_children([child.native])
 
     def refresh(self):
         intrinsic = self.interface.intrinsic
@@ -169,3 +173,38 @@ def refresh(self):
 
         intrinsic.width = self.scale_out_horizontal(intrinsic.width)
         intrinsic.height = self.scale_out_vertical(intrinsic.height)
+
+
+class DeferredMount:
+    """Mount children to a widget once the widget itself is mounted.
+
+    Inject this mixin in to the MRO for native Textual widgets that support children.
+
+    Textual does not permit mounting widgets on to a widget that is not itself already
+    mounted. As such, when a child is added to an unmounted widget, a callback is added
+    that can mount it. This mixin runs those callbacks once the widget is mounted.
+    """
+
+    def __init__(self, *children: TextualWidget):
+        super().__init__(*children)
+        self._on_mount_callback: OrderedDict[Widget, Callable] = OrderedDict()
+
+    def mount(self, widget, **kwargs):
+        if self.is_attached:
+            super().mount(widget, **kwargs)
+        else:
+            self._on_mount_callback[widget] = lambda: self.mount(widget)
+
+    def remove_children(self, widgets):
+        for widget in widgets:
+            try:
+                self._on_mount_callback.pop(widget)
+            except KeyError:
+                super().remove_children([widget])
+
+    def on_mount(self):
+        """Textual calls this hook when the widget is mounted."""
+        if self._on_mount_callback:
+            for child, callback in self._on_mount_callback.copy().items():
+                callback()
+                self._on_mount_callback.pop(child)

textual/src/toga_textual/widgets/box.py

@@ -3,12 +3,16 @@
 from textual.containers import Container as TextualContainer
 from toga.style.pack import ROW
 
-from .base import Widget
+from .base import DeferredMount, Widget
+
+
+class TogaContainer(DeferredMount, TextualContainer):
+    """Textual container that supports deferred children mounting."""
 
 
 class Box(Widget):
     def create(self):
-        self.native = TextualContainer()
+        self.native = TogaContainer()
 
     def set_bounds(self, x, y, width, height):
         super().set_bounds(x, y, width, height)