[iOS] Restore Flutter text focus after native responder recovery - #191781

When iOS restores first responder to a text input view whose connection it
had reported closed, flutter#191781 now tells the framework, which re-requests
focus from TextInputClient.onFocusReceived. The client then attaches a new
connection, and that connection carries a new client id.

iOS delivers the text picked from an AutoFill sheet such as "Hide My Email"
immediately after restoring first responder, using the client id it already
holds. That delivery arrives before a newly attached connection exists, so
the client id check in TextInput._handleTextInputInvocation discards it and
the field stays empty.

Take the reported connection back instead of letting a new one attach, so
the id the platform is using stays valid and the text is applied.

Fixes flutter#157495

…tter#191781)

Fixes flutter#189051

On iOS 26, UIKit resigns the native text responder during a quick app
switch and restores it on return. The engine reports the resignation to
the framework as `onConnectionClosed` but never reports the restoration,
so the keyboard comes back while every `FocusNode` stays unfocused and
typing goes nowhere.

I filed that issue, [together with a sample app that uses nothing but
plain `TextField`
widgets.](https://github.com/hoyahozz/ios-keyboard-stuck-after-app-switch)
### Root cause

Instrumented unpatched engine, iPhone 17 / iOS 26.6 :

```text
18:54:13.415  resignFirstResponder client=1
18:54:13.420  framework focus cleared (onConnectionClosed)
18:54:13.488  AppLifecycleState.paused
18:54:13.783  becomeFirstResponder client=1      <-- UIKit restores the same view
18:54:14.042  AppLifecycleState.resumed
18:54:14.043  all Flutter text fields focus=false
18:54:14.272  viewInsets.bottom=physical=924.0, logical=308.0
```

The `becomeFirstResponder` never reaches Dart. There is no reverse
message for "UIKit restored the same view", so only the framework half
stays down.

I cannot see UIKit's internals, so I compared behaviour instead. A
standalone UIKit app with a single `UITextField` and no Flutter, built
once and installed on both devices, produced this sequence only on iOS
26 :

- iPhone 15 Pro / iOS 18.1.1 : the field stays first responder across
the switch, with no resignation.
- iPhone 17 / iOS 26.6 : `willResignActive → resignFirstResponder →
didEndEditing`, then `willEnterForeground → becomeFirstResponder →
didBeginEditing`.

Plain UIKit recovers on its own, because `didBeginEditing` puts the
field back into editing. Flutter has no equivalent signal. I cannot say
why UIKit does this and Apple does not document it, so I treated it as a
behaviour difference observed from **iOS 26** onward and fixed the
Flutter side of the boundary, which does not depend on the reason.

### The fix

`FlutterTextInputView` now tracks *why* its responder state changed, and
sends the existing `TextInputClient.onFocusReceived` when UIKit restores
it.

- A non-framework `resignFirstResponder` on a valid client arms a
one-shot restore flag.
- A real `false → true` restoration of the same view consumes the flag
and sends one notification, so it fires at most once per cycle.
- `TextInput.show`, `TextInput.hide`, `TextInput.finishAutofillContext`
and `TextInput.setClient` cancel it. `showTextInput`, `hideTextInput`
and `triggerAutofillSave` now call `becomeFirstResponderFromFramework` /
`resignFirstResponderFromFramework`, which is how framework-driven
changes are kept out of the classification.
- Interactive keyboard dismissal never arms it.
- `onConnectionClosed` still fires unchanged, so real handoffs to native
inputs and system menus are unaffected.
- No framework changes. `TextInput` still matches the client id, and
`EditableTextState.onFocusReceived` still checks `mounted`, `_hasFocus`
and `canRequestFocus`, so a stale client cannot steal focus.

The guarantee is narrow : *if UIKit restores the same responder before
the framework declares a new input state, the existing Dart guards get
exactly one chance to accept it.*

### Tests

New Objective-C tests cover :

- one notification per platform `resign → become` cycle
- no notification when either side of the transition came from the
framework
- interactive keyboard dismissal never arming
- `setClient` cancelling a pending restoration, for both a new and a
repeated id
- `show` / `hide` / `finishAutofillContext` going through the wrappers
- the delegate serialising to `TextInputClient.onFocusReceived`

### Validation on device

iPhone 17 / iOS 26.6, using the sample app from the issue. Same app
source and same Flutter revision in both runs. Before, the keyboard
returns but no field is focused and typing goes nowhere; after, the same
field keeps focus across the switch.

#### Before


https://github.com/user-attachments/assets/b70e4875-7737-46cb-a3db-738817f5b04c

#### After


https://github.com/user-attachments/assets/045efe24-0cfd-458b-a567-7fe8fd9def73



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responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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---------

Co-authored-by: LongCatIsLooong <[email protected]>