diff --git a/Signal/src/ViewControllers/ConversationView/ConversationCollectionView.h b/Signal/src/ViewControllers/ConversationView/ConversationCollectionView.h
index d6d34b35c..419c4574e 100644
--- a/Signal/src/ViewControllers/ConversationView/ConversationCollectionView.h
+++ b/Signal/src/ViewControllers/ConversationView/ConversationCollectionView.h
@@ -1,13 +1,13 @@
 //
-//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2019 Open Whisper Systems. All rights reserved.
 //
 
 NS_ASSUME_NONNULL_BEGIN
 
 @protocol ConversationCollectionViewDelegate <NSObject>
 
-- (void)collectionViewWillChangeLayout;
-- (void)collectionViewDidChangeLayout;
+- (void)collectionViewWillChangeSizeFrom:(CGSize)oldSize to:(CGSize)newSize;
+- (void)collectionViewDidChangeSizeFrom:(CGSize)oldSize to:(CGSize)newSize;
 
 @end
 
diff --git a/Signal/src/ViewControllers/ConversationView/ConversationCollectionView.m b/Signal/src/ViewControllers/ConversationView/ConversationCollectionView.m
index a2efe2cf9..a6aad672f 100644
--- a/Signal/src/ViewControllers/ConversationView/ConversationCollectionView.m
+++ b/Signal/src/ViewControllers/ConversationView/ConversationCollectionView.m
@@ -1,5 +1,5 @@
 //
-//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2019 Open Whisper Systems. All rights reserved.
 //
 
 #import "ConversationCollectionView.h"
@@ -24,13 +24,15 @@ NS_ASSUME_NONNULL_BEGIN
         // hacks in the conversation view's code.
         return;
     }
-    BOOL isChanging = !CGSizeEqualToSize(frame.size, self.frame.size);
+    CGSize oldSize = self.frame.size;
+    CGSize newSize = frame.size;
+    BOOL isChanging = !CGSizeEqualToSize(oldSize, newSize);
     if (isChanging) {
-        [self.layoutDelegate collectionViewWillChangeLayout];
+        [self.layoutDelegate collectionViewWillChangeSizeFrom:oldSize to:newSize];
     }
     [super setFrame:frame];
     if (isChanging) {
-        [self.layoutDelegate collectionViewDidChangeLayout];
+        [self.layoutDelegate collectionViewDidChangeSizeFrom:oldSize to:newSize];
     }
 }
 
@@ -48,13 +50,15 @@ NS_ASSUME_NONNULL_BEGIN
         // hacks in the conversation view's code.
         return;
     }
-    BOOL isChanging = !CGSizeEqualToSize(bounds.size, self.bounds.size);
+    CGSize oldSize = self.bounds.size;
+    CGSize newSize = bounds.size;
+    BOOL isChanging = !CGSizeEqualToSize(oldSize, newSize);
     if (isChanging) {
-        [self.layoutDelegate collectionViewWillChangeLayout];
+        [self.layoutDelegate collectionViewWillChangeSizeFrom:oldSize to:newSize];
     }
     [super setBounds:bounds];
     if (isChanging) {
-        [self.layoutDelegate collectionViewDidChangeLayout];
+        [self.layoutDelegate collectionViewDidChangeSizeFrom:oldSize to:newSize];
     }
 }
 
diff --git a/Signal/src/ViewControllers/ConversationView/ConversationViewController.m b/Signal/src/ViewControllers/ConversationView/ConversationViewController.m
index d07c6d6c4..fa93b149d 100644
--- a/Signal/src/ViewControllers/ConversationView/ConversationViewController.m
+++ b/Signal/src/ViewControllers/ConversationView/ConversationViewController.m
@@ -563,6 +563,9 @@ typedef enum : NSUInteger {
     self.layout.delegate = self;
     // We use the root view bounds as the initial frame for the collection
     // view so that its contents can be laid out immediately.
+    //
+    // TODO: To avoid relayout, it'd be better to take into account safeAreaInsets,
+    //       but they're not yet set when this method is called.
     _collectionView =
         [[ConversationCollectionView alloc] initWithFrame:self.view.bounds collectionViewLayout:self.layout];
     self.collectionView.layoutDelegate = self;
@@ -1621,11 +1624,8 @@ typedef enum : NSUInteger {
 {
     OWSLogInfo(@"didChangePreferredContentSize");
 
-    // Evacuate cached cell sizes.
-    for (id<ConversationViewItem> viewItem in self.viewItems) {
-        [viewItem clearCachedLayoutState];
-    }
-    [self resetContentAndLayout];
+    [self resetForSizeOrOrientationChange];
+
     [self.inputToolbar updateFontSizes];
 }
 
@@ -4218,18 +4218,18 @@ typedef enum : NSUInteger {
 
 #pragma mark - ConversationCollectionViewDelegate
 
-- (void)collectionViewWillChangeLayout
+- (void)collectionViewWillChangeSizeFrom:(CGSize)oldSize to:(CGSize)newSize
 {
     OWSAssertIsOnMainThread();
 }
 
-- (void)collectionViewDidChangeLayout
+- (void)collectionViewDidChangeSizeFrom:(CGSize)oldSize to:(CGSize)newSize
 {
     OWSAssertIsOnMainThread();
 
     [self updateLastVisibleSortId];
-    self.conversationStyle.viewWidth = self.collectionView.width;
-    [self.collectionView.collectionViewLayout invalidateLayout];
+
+    [self resetForSizeOrOrientationChange];
 }
 
 #pragma mark - View Items
@@ -4764,13 +4764,14 @@ typedef enum : NSUInteger {
 {
     [super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];
 
-    self.conversationStyle.viewWidth = size.width;
-
-    for (id<ConversationViewItem> viewItem in self.viewItems) {
-        [viewItem clearCachedLayoutState];
-    }
-
-    [self resetContentAndLayout];
+    __weak ConversationViewController *weakSelf = self;
+    [coordinator
+        animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext> context) {
+            // Do nothing.
+        }
+        completion:^(id<UIViewControllerTransitionCoordinatorContext> context) {
+            [weakSelf resetForSizeOrOrientationChange];
+        }];
 
     // TODO: Ensure scroll state continuity?
 }
@@ -4783,6 +4784,23 @@ typedef enum : NSUInteger {
     [self ensureBannerState];
 }
 
+- (void)resetForSizeOrOrientationChange
+{
+    self.scrollContinuity = kScrollContinuityBottom;
+
+    self.conversationStyle.viewWidth = self.collectionView.width;
+    // Evacuate cached cell sizes.
+    for (id<ConversationViewItem> viewItem in self.viewItems) {
+        [viewItem clearCachedLayoutState];
+    }
+    [self.collectionView.collectionViewLayout invalidateLayout];
+    [self.collectionView reloadData];
+    if (self.viewHasEverAppeared) {
+        // Try to update the lastKnownDistanceFromBottom; the content size may have changed.
+        [self updateLastKnownDistanceFromBottom];
+    }
+}
+
 @end
 
 NS_ASSUME_NONNULL_END