fix(iOS): Fix scroll issues related to keyboard dismissal (#176) (#399)

- We only need the KeyboardWillHide event to fix this problem, and we only need it for ios 11+
- Enable the fix for ios 11+, not only ios 12+. It was probably only 12+ because no devices have a max OS of 11, so no dev probably confirmed the problem's existence (or non-existence) on 11, myself included.  (But the functionality that started this issue chain began in ios 11.)
- Do some math to make sure the offset when the keyboard goes down doesn't result in empty white space being displayed.
- This solution can potentially be removed when apple finally releases the fix for the issue discussed here: apache/cordova-ios#417 (comment)

src/ios/CDVWKWebViewEngine.m

@@ -127,8 +127,6 @@ @implementation CDVWKWebViewEngine
 
 @synthesize engineWebView = _engineWebView;
 
-NSTimer *timer;
-
 - (instancetype)initWithFrame:(CGRect)frame
 {
     self = [super init];
@@ -410,16 +408,13 @@ - (void)pluginInitialize
      selector:@selector(onSocketError:)
      name:@"socketInUseError" object:nil];
 
-    [[NSNotificationCenter defaultCenter]
-     addObserver:self
-     selector:@selector(keyboardWillHide)
-     name:UIKeyboardWillHideNotification object:nil];
-
-    [[NSNotificationCenter defaultCenter]
-     addObserver:self
-     selector:@selector(keyboardWillShow)
-     name:UIKeyboardWillShowNotification object:nil];
-
+    if (@available(iOS 11.0, *)) {
+        // For keyboard dismissal leaving viewport shifted (can potentially be removed when apple releases the fix for the issue discussed here: https://github.com/apache/cordova-ios/issues/417#issuecomment-423340885)
+        [[NSNotificationCenter defaultCenter]
+         addObserver:self
+         selector:@selector(keyboardWillHide)
+         name:UIKeyboardWillHideNotification object:nil];
+    }
 
     NSLog(@"Using Ionic WKWebView");
 
@@ -531,31 +526,26 @@ - (void)onAppWillEnterForeground:(NSNotification *)notification {
     }
 }
 
-
 -(void)keyboardWillHide
 {
-    if (@available(iOS 12.0, *)) {
-        timer = [NSTimer scheduledTimerWithTimeInterval:0 target:self selector:@selector(keyboardDisplacementFix) userInfo:nil repeats:false];
-        [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
-    }
-}
-
--(void)keyboardWillShow
-{
-    if (timer != nil) {
-        [timer invalidate];
+    if (@available(iOS 11.0, *)) {
+        // For keyboard dismissal leaving viewport shifted (can potentially be removed when apple releases the fix for the issue discussed here: https://github.com/apache/cordova-ios/issues/417#issuecomment-423340885)
+        UIScrollView * scrollView = self.webView.scrollView;
+        // Calculate some vars for convenience
+        CGFloat contentLengthWithInsets = scrollView.contentSize.height + scrollView.adjustedContentInset.top + scrollView.adjustedContentInset.bottom;
+        CGFloat contentOffsetY = scrollView.contentOffset.y;
+        CGFloat screenHeight = scrollView.frame.size.height;
+        CGFloat maxAllowedOffsetY = fmax(contentLengthWithInsets - screenHeight, 0); // 0 is for the case where content is shorter than screen
+
+        // If the keyboard allowed the user to get to an offset beyond the max
+        if (contentOffsetY > maxAllowedOffsetY) {
+            // Reset the scroll to the max allowed so that there is no additional empty white space at the bottom where the keyboard occupied!
+            CGPoint bottomOfPage = CGPointMake(scrollView.contentOffset.x, maxAllowedOffsetY);
+            [scrollView setContentOffset:bottomOfPage];
+        }
     }
 }
 
--(void)keyboardDisplacementFix
-{
-    // https://stackoverflow.com/a/9637807/824966
-    [UIView animateWithDuration:.25 animations:^{
-        self.webView.scrollView.contentOffset = CGPointMake(0, 0);
-    }];
-
-}
-
 - (void)onSocketError:(NSNotification *)notification {
     [self loadErrorPage:nil];
 }