docs: add 0.85.3 changelog entry for #6527 FLET_APP_HIDDEN fix (#6561)

PR #6527 (FLET_APP_HIDDEN honored in Windows/Linux desktop runners)
was merged before the 0.85.3 release was cut but was not listed in
either CHANGELOG.md or packages/flet/CHANGELOG.md. The Dart-side
changelog also incorrectly stated "No changes in the `flet` Dart
package" for 0.85.3 despite window.dart changes from #6527.

Author: FeodorFitsner

CHANGELOG.md

@@ -13,6 +13,7 @@
 
 * Fix `flet.Router`'s default `on_view_pop` navigating to the wrong URL when an `outlet=True` layout sits between two views in `manage_views=True` mode. Popping such a view now targets the previous view entry's resolved URL — skipping outlet layouts and componentless grouping routes — instead of `chain[-2]`, which could equal the current view's URL and strand the page route, making the next navigation to it a no-op ([#6533](https://github.com/flet-dev/flet/pull/6533)) by @FeodorFitsner.
 * Fix `flet-audio.Audio.play()`/`seek()` timing out when replaying after playback had completed: under the default `ReleaseMode.RELEASE` the source is freed on completion and is now re-prepared on replay ([#6536](https://github.com/flet-dev/flet/issues/6536), [#6538](https://github.com/flet-dev/flet/pull/6538)) by @ndonkoHenri.
+* Fix `ft.run(view=ft.AppView.FLET_APP_HIDDEN)` briefly flashing the native window in the top-left corner during Windows desktop startup. The Windows runner now respects `FLET_HIDE_WINDOW_ON_START` and skips the first-frame `Show()` call so the window stays hidden until `page.window.visible = True`, matching the Linux desktop behavior; the same fix is applied to the `flet build windows` template runner so generated apps behave consistently. On Linux, pre-show window placement actions (`page.window.center()`, `page.window.alignment`) are now deferred until the window first becomes visible to avoid an analogous flash, and the window's `focused` state is preserved when a `prevent_close` handler cancels a close attempt ([#5897](https://github.com/flet-dev/flet/issues/5897), [#5914](https://github.com/flet-dev/flet/issues/5914), [#6527](https://github.com/flet-dev/flet/pull/6527)) by @ihmily.
 
 ## 0.85.2
 

packages/flet/CHANGELOG.md

@@ -1,6 +1,8 @@
 ## 0.85.3
 
-_No changes in the `flet` Dart package; version bumped for release coordination with a `flet.Router` view-pop fix on the Python side._
+### Bug fixes
+
+* Defer pre-show window placement on Linux (`centerWindow()`, `setWindowAlignment()`) until the window first becomes visible, so `page.window.center()` / `page.window.alignment` set before `page.window.visible = True` no longer flash the window during startup. Also preserve the `focused` state when a `prevent_close` handler cancels a close attempt ([#5897](https://github.com/flet-dev/flet/issues/5897), [#5914](https://github.com/flet-dev/flet/issues/5914), [#6527](https://github.com/flet-dev/flet/pull/6527)) by @ihmily.
 
 ## 0.85.2