ConversationView first load avoids redundant layout

We are laying out the collection view, invalidating the layout, and then
laying out the collection view again on first appearance of the
conversation view. This is quite expensive - removing it shaves off
about 30% of load time.

// FREEBIE

Signal/src/ViewControllers/ConversationView/ConversationViewController.m

@@ -4461,7 +4461,14 @@ typedef enum : NSUInteger {
         // Snapshot the scroll state by measuring the "distance from top of view to
         // bottom of content"; if the mapping's "window" size grows, it will grow
         // _upward_.
-        CGFloat viewTopToContentBottom = self.safeContentHeight - self.collectionView.contentOffset.y;
+        CGFloat viewTopToContentBottom = 0;
+        if ([self.collectionView.collectionViewLayout isKindOfClass:[ConversationViewLayout class]]) {
+            ConversationViewLayout *conversationViewLayout
+                = (ConversationViewLayout *)self.collectionView.collectionViewLayout;
+            if (conversationViewLayout.hasLayout) {
+                viewTopToContentBottom = self.safeContentHeight - self.collectionView.contentOffset.y;
+            }
+        }
 
         NSUInteger oldCellCount = [self.messageMappings numberOfItemsInGroup:self.thread.uniqueId];
 

Signal/src/ViewControllers/ConversationView/ConversationViewLayout.h

@@ -37,7 +37,7 @@ typedef NS_ENUM(NSInteger, ConversationViewLayoutAlignment) {
 @interface ConversationViewLayout : UICollectionViewLayout
 
 @property (nonatomic, weak) id<ConversationViewLayoutDelegate> delegate;
-
+@property (nonatomic, readonly) BOOL hasLayout;
 @property (nonatomic, readonly) int contentWidth;
 
 @end