Deprecate FletApp.show_app_startup_screen / app_startup_screen_message via @deprecated

Use the standard @deprecated decorator (like DragTargetEvent.x/y) instead of a
docstring note: the two settings are now deprecated property getters/setters
that proxy to boot_screen_options (spinner_size / startup_message). Since they
are properties, they're no longer constructor kwargs — set them as attributes
or migrate to boot_screen_options.

Drop the now-dead Dart legacy fallback in flet_app_control.dart and the unused
legacyBootScreenOptions helper (the legacy fields are no longer serialized).

Author: FeodorFitsner

packages/flet/lib/src/controls/flet_app_control.dart

@@ -5,7 +5,6 @@ import '../flet_app_errors_handler.dart';
 import '../flet_backend.dart';
 import '../models/control.dart';
 import '../utils/numbers.dart';
-import '../widgets/boot_screen.dart';
 import 'base_controls.dart';
 
 class FletAppControl extends StatefulWidget {
@@ -37,13 +36,9 @@ class _FletAppControlState extends State<FletAppControl> {
     var reconnectTimeoutMs = widget.control.getInt("reconnect_timeout_ms");
     var bootScreenName = widget.control.getString("boot_screen_name", "flet")!;
     var rawBootScreenOptions = widget.control.get("boot_screen_options");
-    // New boot_screen_options take precedence; otherwise fall back to the
-    // deprecated show/message startup-screen settings.
     var bootScreenOptions = rawBootScreenOptions is Map
         ? Map<String, dynamic>.from(rawBootScreenOptions)
-        : legacyBootScreenOptions(
-            widget.control.getBool("show_app_startup_screen", false)!,
-            widget.control.getString("app_startup_screen_message"));
+        : <String, dynamic>{};
     var appErrorMessage = widget.control.getString("app_error_message");
 
     return LayoutControl(

packages/flet/lib/src/widgets/boot_screen.dart

@@ -23,13 +23,3 @@ Widget resolveBootScreen({
   }
   return FletBootScreen(options: options, status: status);
 }
-
-/// Builds built-in "flet" boot screen [options] from the legacy
-/// show/message startup-screen settings, for backward compatibility.
-Map<String, dynamic> legacyBootScreenOptions(bool show, String? message) {
-  if (!show) return const {};
-  return {
-    "spinner_size": 30,
-    if (message != null && message.isNotEmpty) "startup_message": message,
-  };
-}

sdk/python/packages/flet/src/flet/controls/core/flet_app.py

@@ -4,6 +4,7 @@
 from flet.controls.base_control import control
 from flet.controls.control_event import ControlEventHandler, Event, EventHandler
 from flet.controls.layout_control import LayoutControl
+from flet.utils.deprecated import deprecated
 
 __all__ = ["FletApp", "FletAppOutputEvent"]
 
@@ -77,22 +78,6 @@ class FletApp(LayoutControl):
     [boot screen docs](https://flet.dev/docs/publish/#boot-screen).
     """
 
-    show_app_startup_screen: bool = False
-    """
-    Whether to show the app startup screen.
-
-    Deprecated: use :attr:`boot_screen_options` instead, e.g.
-    `boot_screen_options={"spinner_size": 30}`.
-    """
-
-    app_startup_screen_message: Optional[str] = None
-    """
-    Message to display on the app startup screen.
-
-    Deprecated: use :attr:`boot_screen_options` instead, e.g.
-    `boot_screen_options={"startup_message": "Loading…"}`.
-    """
-
     app_error_message: Optional[str] = None
     """
     Template message to display when the app fails to load.
@@ -113,3 +98,59 @@ class FletApp(LayoutControl):
     `force_pyodide=True`; root-level Pyodide pages have nowhere to
     bubble the event.
     """
+
+    @property
+    @deprecated(
+        reason="Use `boot_screen_options` instead, e.g. "
+        "boot_screen_options={'spinner_size': 30}.",
+        version="0.86.0",
+        delete_version="0.89.0",
+    )
+    def show_app_startup_screen(self) -> bool:
+        """
+        Whether to show the app startup screen.
+        """
+        return bool((self.boot_screen_options or {}).get("spinner_size", 0))
+
+    @show_app_startup_screen.setter
+    @deprecated(
+        reason="Use `boot_screen_options` instead, e.g. "
+        "boot_screen_options={'spinner_size': 30}.",
+        version="0.86.0",
+        delete_version="0.89.0",
+    )
+    def show_app_startup_screen(self, value: bool) -> None:
+        options = dict(self.boot_screen_options or {})
+        if value:
+            options.setdefault("spinner_size", 30)
+        else:
+            options.pop("spinner_size", None)
+        self.boot_screen_options = options
+
+    @property
+    @deprecated(
+        reason="Use `boot_screen_options` instead, e.g. "
+        "boot_screen_options={'startup_message': '...'}.",
+        version="0.86.0",
+        delete_version="0.89.0",
+    )
+    def app_startup_screen_message(self) -> Optional[str]:
+        """
+        Message to display on the app startup screen.
+        """
+        return (self.boot_screen_options or {}).get("startup_message")
+
+    @app_startup_screen_message.setter
+    @deprecated(
+        reason="Use `boot_screen_options` instead, e.g. "
+        "boot_screen_options={'startup_message': '...'}.",
+        version="0.86.0",
+        delete_version="0.89.0",
+    )
+    def app_startup_screen_message(self, value: Optional[str]) -> None:
+        options = dict(self.boot_screen_options or {})
+        if value is not None:
+            options["startup_message"] = value
+        else:
+            options.pop("startup_message", None)
+        self.boot_screen_options = options