Improve scroll state continuity during conversation view rotations.

Signal/src/ViewControllers/ConversationView/ConversationViewController.m

@@ -4347,10 +4347,24 @@ typedef enum : NSUInteger {
     targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset
 {
     if (self.scrollContinuity == kScrollContinuityBottom && self.lastKnownDistanceFromBottom) {
+        NSValue *_Nullable contentOffset =
+            [self contentOffsetForLastKnownDistanceFromBottom:self.lastKnownDistanceFromBottom.floatValue];
+        if (contentOffset) {
+            proposedContentOffset = contentOffset.CGPointValue;
+        }
+    }
+
+    return proposedContentOffset;
+}
+
+// We use this hook to ensure scroll state continuity.  As the collection
+// view's content size changes, we want to keep the same cells in view.
+- (nullable NSValue *)contentOffsetForLastKnownDistanceFromBottom:(CGFloat)lastKnownDistanceFromBottom
+{
     // Adjust the content offset to reflect the "last known" distance
     // from the bottom of the content.
     CGFloat contentOffsetYBottom = self.maxContentOffsetY;
-        CGFloat contentOffsetY = contentOffsetYBottom - MAX(0, self.lastKnownDistanceFromBottom.floatValue);
+    CGFloat contentOffsetY = contentOffsetYBottom - MAX(0, lastKnownDistanceFromBottom);
     CGFloat minContentOffsetY;
     if (@available(iOS 11, *)) {
         minContentOffsetY = -self.collectionView.safeAreaInsets.top;
@@ -4358,10 +4372,7 @@ typedef enum : NSUInteger {
         minContentOffsetY = 0.f;
     }
     contentOffsetY = MAX(minContentOffsetY, contentOffsetY);
-        proposedContentOffset.y = contentOffsetY;
-    }
-
-    return proposedContentOffset;
+    return [NSValue valueWithCGPoint:CGPointMake(0, contentOffsetY)];
 }
 
 #pragma mark - Scroll State
@@ -4811,18 +4822,34 @@ typedef enum : NSUInteger {
 {
     [super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];
 
+    // Update and snapshot the "last known distance from bottom".
+    [self updateLastKnownDistanceFromBottom];
+    NSNumber *_Nullable lastKnownDistanceFromBottom = self.lastKnownDistanceFromBottom;
+
     __weak ConversationViewController *weakSelf = self;
     [coordinator
         animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext> context) {
             // Do nothing.
         }
         completion:^(id<UIViewControllerTransitionCoordinatorContext> context) {
+            ConversationViewController *strongSelf = weakSelf;
+            if (!strongSelf) {
+                return;
+            }
+
             // When transition animation is complete, update layout to reflect
             // new size.
-            [weakSelf resetForSizeOrOrientationChange];
-        }];
+            [strongSelf resetForSizeOrOrientationChange];
 
-    // TODO: Ensure scroll state continuity?
+            if (lastKnownDistanceFromBottom) {
+                NSValue *_Nullable contentOffsetValue =
+                    [strongSelf contentOffsetForLastKnownDistanceFromBottom:lastKnownDistanceFromBottom.floatValue];
+                if (contentOffsetValue) {
+                    CGPoint contentOffset = contentOffsetValue.CGPointValue;
+                    [strongSelf.collectionView setContentOffset:contentOffset animated:NO];
+                }
+            }
+        }];
 }
 
 - (void)traitCollectionDidChange:(nullable UITraitCollection *)previousTraitCollection