[ios][platform_view] workaround for non-tappable webview

[hellohuanlin]

This is pretty tricky bug - I suspect that because Apple's internal recognizer caching an outdated state of our delaying recognizer.

The conflict happens between WebKit and platform view's recognizers. It happens to all plugins that uses a WKWebView, or any view that has a similar (unknown) gesture setup.

This fix has to be in the engine (rather than the plugin), because the plugin itself knows nothing about the existence of our delaying recognizer.

Here are the steps of my research for future reference:

1. The bug only happens when the overlay flutter widgets blocks the gestures for the platform view (e.g. tap on the platform view area when a flutter context menu is displayed). When the bug happens, WKWebView's link doesn't work anymore, however, the link can still be highlighted when tapped.

2. When I remove the delaying recognizer from platform view, the link became clickable again. This means that the bug is related to some conflict between WKWebView's internal recognizers and our delaying recognizer. This also means that it is not possible to fix this issue at plugin level, which knows nothing about the delaying recognizer.

3. When we tap on the web view when context menu is displayed, blockGesture will be called, which simply toggles delaying recognizer's state from possible to ended state, meaning it should block all recognizers from the current gesture (and it correctly did so). Then I use dispatch_async and check the state again, and confirmed the state is correctly reset to possible state.

4. Subsequent tap on web view triggers acceptGesture, which turns the possible state into failed state. This subsequent tap only highlights the link, but not activate the link. This suggests that some internal web kit recognizer that handles the highlight sees the failed state of delaying recognizer (which is correct), but the recognizer that handles the link activation probably sees the stale state of possible or ended (which is outdated old state).

5. So the solution is trying to make the recognizer "see" the updated state rather than the cached old state.

6. I tried recreating a new delaying recognizer when blockGesture is called:

- blockGesture {
  delayingRecognizer.state = .ended
  dispatch_async {
    // force re-create the delaying recognizer
  }
}

This fixed the link activation bug, however, when opening the context menu again, the gesture is not blocked anymore. This means web kit internal recognizers likely cache the old delaying recognizer for state update, thus the new instance of delaying recognizer won't work anymore. So we can't change the instance. However, it's a good experiment that confirms my hypothesis that some internal webkit recognizer caches the outdated state of delaying recognizer.

7. For the above code, rather than using dispatch_async, I also tried dispatch_after, and it turns out that it only works if the dispatch_after delay is 0 - even if the delay is much smaller than 1 frame's time (16.7ms), it doesn't work. This means the state checking happens either at the end of the current run loop, or beginning of the next run loop. (not too important information, but it helps me better understand how UIKit works).

8. So from 6, we know that we have to keep the original instance of delaying recognizer. I tried toggling recognizer.enabled, it didn't work. I also tried inserting a dummy recognizer, it didn't work. Neither approach triggers the state "refresh" for those webkit internal recognizers.

9. I tried removing and adding back the delaying recognizer, and it just worked! This means that removing and adding back the delaying recognizer probably triggered UIKit to refresh the states for all its related recognizers (i.e. those recognizers either blocking or being blocked by the delaying recognizer), hence getting the updated state.

List which issues are fixed by this PR. You must list at least one issue.

Fixes flutter/flutter#158961

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[jmagman]

The code LGTM, the behavior RSLGTM but it sounds like you thoroughly tested. Hopefully this gets fixed in a future version of iOS and we can fallback to the old behavior.

Also, you have a merge conflict.

[hellohuanlin]

@cbracken kind reminder, in case any comments before i land

[hellohuanlin]

i think i will verify with 18.2 RC which is out today before landing it

[cbracken]