[iOS] Increase offset for infinite feed triggering

Increase offset to fit ~2 cards before triggering infinite feed. Compute
initial NTP scroll height in layout to only trigger infinite feed if the
feed has loaded.

Bug: 1085419
Change-Id: I9fccd5f135382d5574ccb6a7ce562339455c7564
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2355294
Commit-Queue: Adam Trudeau-Arcaro <adamta@google.com>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798296}

ios/chrome/browser/ui/content_suggestions/content_suggestions_layout.h

@@ -16,6 +16,9 @@
 // The cached scroll position of the NTP.
 @property(nonatomic, assign) CGFloat offset;
 
+// The total scroll height of the NTP.
+@property(nonatomic, assign) CGFloat ntpHeight;
+
 // Creates layout with |offset| as additional height. Allows the view's height
 // to be increased enough to maintain the scroll position. Only needed if
 // Discover feed is enabled.

ios/chrome/browser/ui/content_suggestions/content_suggestions_layout.mm

@@ -30,9 +30,11 @@
   // The minimum height for the collection view content should be the height of
   // the header plus the height of the collection view minus the height of the
   // NTP bottom bar. This allows the Most Visited cells to be scrolled up to the
-  // top of the screen.
-  CGFloat minimumHeight = collectionViewHeight + headerHeight -
-                          ntp_header::kScrolledToTopOmniboxBottomMargin;
+  // top of the screen. Also computes the total NTP scrolling height for
+  // Discover infinite feed.
+  self.ntpHeight = collectionViewHeight + headerHeight;
+  CGFloat minimumHeight =
+      self.ntpHeight - ntp_header::kScrolledToTopOmniboxBottomMargin;
   CGFloat topSafeArea = self.collectionView.safeAreaInsets.top;
   if (!IsRegularXRegularSizeClass(self.collectionView)) {
     CGFloat toolbarHeight =
@@ -40,8 +42,10 @@
             ? ToolbarExpandedHeight([UIApplication sharedApplication]
                                         .preferredContentSizeCategory)
             : 0;
-    minimumHeight -=
+    CGFloat additionalHeight =
         toolbarHeight + topSafeArea + self.collectionView.contentInset.bottom;
+    minimumHeight -= additionalHeight;
+    self.ntpHeight += additionalHeight;
   }
 
   CGSize contentSize = [super collectionViewContentSize];

ios/chrome/browser/ui/content_suggestions/content_suggestions_view_controller.mm

@@ -52,7 +52,7 @@ const CGFloat kMostVisitedBottomMargin = 13;
 const CGFloat kCardBorderRadius = 11;
 const CGFloat kDiscoverFeedContentWith = 430;
 // Value representing offset from bottom of the page to trigger pagination.
-const CGFloat kPaginationOffset = 100;
+const CGFloat kPaginationOffset = 400;
 // Height for the Discover Feed section header.
 const CGFloat kDiscoverFeedFeaderHeight = 30;
 }
@@ -697,7 +697,15 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
   if (IsDiscoverFeedEnabled() && self.contentSuggestionsEnabled) {
     float scrollPosition =
         scrollView.contentOffset.y + scrollView.frame.size.height;
-    if (scrollPosition >= scrollView.contentSize.height - kPaginationOffset) {
+    // Check if view is bouncing to ignore overscoll positions for infinite feed
+    // triggering.
+    BOOL isBouncing =
+        (scrollView.contentOffset.y >=
+         (scrollView.contentSize.height - scrollView.bounds.size.height));
+    ContentSuggestionsLayout* layout = static_cast<ContentSuggestionsLayout*>(
+        self.collectionView.collectionViewLayout);
+    if (scrollPosition > scrollView.contentSize.height - kPaginationOffset &&
+        scrollPosition > layout.ntpHeight && !isBouncing) {
       [self.handler loadMoreFeedArticles];
     }
   }