Don't orphan SplitContainer content when the assignment is invalid

changes/4631.bugfix.md

@@ -0,0 +1 @@
+A failed `SplitContainer.content` assignment no longer orphans the currently-displayed widgets.

core/src/toga/widgets/splitcontainer.py

@@ -97,11 +97,6 @@ def content(self) -> list[SplitContainerContentT]:
 
     @content.setter
     def content(self, content: Sequence[SplitContainerContentT]) -> None:
-        for old_content in self._content:
-            if old_content is not None:
-                old_content.app = None
-                old_content.window = None
-
         try:
             if len(content) != 2:
                 raise TypeError()
@@ -131,6 +126,14 @@ def content(self, content: Sequence[SplitContainerContentT]) -> None:
             _content.append(widget)
             flex.append(flex_value)
 
+        # Only detach the old content once the new content is fully validated,
+        # so a failed assignment doesn't orphan the widgets currently shown.
+        for old_content in self._content:
+            if old_content is not None:
+                old_content.app = None
+                old_content.window = None
+
+        for widget in _content:
             if widget:
                 widget.app = self.app
                 widget.window = self.window

core/tests/widgets/test_splitcontainer.py

@@ -412,6 +412,24 @@ def test_set_content_invalid(splitcontainer, content, message):
         splitcontainer.content = content
 
 
+def test_set_content_invalid_keeps_previous_content(content1, content2):
+    """A failed content assignment must not orphan the widgets currently shown
+    (regression: the old content was detached before validation)."""
+    app = toga.App("Test App", "org.beeware.toga.splitcontainer-test")
+    window = toga.Window()
+    splitcontainer = toga.SplitContainer(content=[content1, content2])
+    window.content = splitcontainer
+
+    assert content1.id in app.widgets
+
+    with pytest.raises(ValueError):
+        splitcontainer.content = [content1]
+
+    # The previous content is still attached and intact
+    assert content1.id in app.widgets
+    assert splitcontainer.content == [content1, content2]
+
+
 def test_direction(splitcontainer):
     """The direction of the splitcontainer can be changed."""