Prime out essential interface attributes before impl initialization

changes/3071.misc.rst

@@ -0,0 +1 @@
+Essential attributes on the widgets' interface are now primed out before initialization of widgets' impl.

core/src/toga/widgets/canvas/canvas.py

@@ -85,10 +85,10 @@ def __init__(
         :param on_alt_release: Initial :any:`on_alt_release` handler.
         :param on_alt_drag: Initial :any:`on_alt_drag` handler.
         """
-        super().__init__(id=id, style=style)
-
         self._context = Context(canvas=self)
 
+        super().__init__(id=id, style=style)
+
         # Set all the properties
         self.on_resize = on_resize
         self.on_press = on_press

core/src/toga/widgets/numberinput.py

@@ -108,8 +108,6 @@ def __init__(
         :param on_change: A handler that will be invoked when the value of the widget
             changes.
         """
-        super().__init__(id=id, style=style)
-
         # The initial setting of min requires calling get_value(),
         # which in turn interrogates min. Prime those values with
         # an empty starting value
@@ -118,6 +116,8 @@ def __init__(
 
         self.on_change = None
 
+        super().__init__(id=id, style=style)
+
         self.readonly = readonly
         self.step = step
         self.min = min

core/src/toga/widgets/optioncontainer.py

@@ -393,10 +393,11 @@ def __init__(
             <OptionContainerContentT>` to display in the OptionContainer.
         :param on_select: Initial :any:`on_select` handler.
         """
-        super().__init__(id=id, style=style)
         self._content = OptionList(self)
         self.on_select = None
 
+        super().__init__(id=id, style=style)
+
         if content is not None:
             for item in content:
                 if isinstance(item, OptionItem):

core/src/toga/widgets/scrollcontainer.py

@@ -40,11 +40,12 @@ def __init__(
         :param on_scroll: Initial :any:`on_scroll` handler.
         :param content: The content to display in the scroll window.
         """
-        super().__init__(id=id, style=style)
 
         self._content: Widget | None = None
         self.on_scroll = None
 
+        super().__init__(id=id, style=style)
+
         # Set all attributes
         self.vertical = vertical
         self.horizontal = horizontal

core/src/toga/widgets/selection.py

@@ -46,13 +46,14 @@ def __init__(
         :param on_change: Initial :any:`on_change` handler.
         :param enabled: Whether the user can interact with the widget.
         """
-        super().__init__(id=id, style=style)
 
         self._items: SourceT | ListSource
 
         self.on_change = None  # needed for _impl initialization
 
         self._accessor = accessor
+        super().__init__(id=id, style=style)
+
         self.items = items
         if value:
             self.value = value

core/src/toga/widgets/splitcontainer.py

@@ -42,8 +42,8 @@ def __init__(
         :param content: Initial :any:`SplitContainer content <SplitContainerContentT>`
             of the container. Defaults to both panels being empty.
         """
-        super().__init__(id=id, style=style)
         self._content: list[SplitContainerContentT] = [None, None]
+        super().__init__(id=id, style=style)
 
         if content:
             self.content = content