Fix O(N²) mount/unmount scans in Session.patch_control (#6651)

* Fix O(N²) mount/unmount scans in Session.patch_control

patch_control checked every removed control against all added controls
(and vice versa) with linear any() scans, making each patch O(N²) in the
number of added/removed controls. When a @ft.component re-renders, the
frozen diff reports every matched control as both removed and re-added,
so components rendering large lists pay this cost on every re-render,
blocking the asyncio event loop for the duration.

Precompute added/removed ID sets once so each membership check is O(1),
reducing the whole function to O(N). Behaviour is unchanged:
will_unmount/did_mount still fire only for controls not present in both
sets, and index management, logging, and message sending are untouched.

* Updating CHANGELOG.md for #6651

Author: davidlawson

CHANGELOG.md

@@ -69,6 +69,7 @@
 * 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.
+* Improve performance of checking added/removed controls in Session.patch_control from O(N²) to O(N) ([#6651](https://github.com/flet-dev/flet/pull/6651)) by @davidlawson.
 
 ## 0.85.2
 

sdk/python/packages/flet/src/flet/messaging/session.py

@@ -247,8 +247,9 @@ def patch_control(
         for c in removed_controls:
             patch_logger.debug("   %s", c)
 
+        added_ids = {added_control._i for added_control in added_controls}
         for removed_control in removed_controls:
-            if not any(added._i == removed_control._i for added in added_controls):
+            if removed_control._i not in added_ids:
                 removed_control.will_unmount()
             self.__index.pop(removed_control._i, None)
 
@@ -264,9 +265,10 @@ def patch_control(
         for ac in added_controls:
             patch_logger.debug("   %s", ac)
 
+        removed_ids = {removed_control._i for removed_control in removed_controls}
         for added_control in added_controls:
             self.__index[added_control._i] = added_control
-            if not any(removed._i == added_control._i for removed in removed_controls):
+            if added_control._i not in removed_ids:
                 added_control.did_mount()
 
     def apply_patch(self, control_id: int, patch: dict[str, Any]):