Preserve scroll state across conversation view layout changes, if possible.

// FREEBIE

Podfile

@@ -5,8 +5,8 @@ target 'Signal' do
     pod 'ATAppUpdater'
     pod 'AxolotlKit',                 git: 'https://github.com/WhisperSystems/SignalProtocolKit.git'
     #pod 'AxolotlKit',                 path: '../SignalProtocolKit'
-    pod 'JSQMessagesViewController',  git: 'https://github.com/WhisperSystems/JSQMessagesViewController.git', branch: 'signal-master'
-    #pod 'JSQMessagesViewController',   path: '../JSQMessagesViewController'
+    #pod 'JSQMessagesViewController',  git: 'https://github.com/WhisperSystems/JSQMessagesViewController.git', branch: 'signal-master'
+    pod 'JSQMessagesViewController',   path: '../JSQMessagesViewController'
     pod 'PureLayout'
     pod 'OpenSSL',                    git: 'https://github.com/WhisperSystems/OpenSSL-Pod'
     pod 'Reachability'

Podfile.lock

@@ -114,7 +114,7 @@ PODS:
 DEPENDENCIES:
   - ATAppUpdater
   - AxolotlKit (from `https://github.com/WhisperSystems/SignalProtocolKit.git`)
-  - JSQMessagesViewController (from `https://github.com/WhisperSystems/JSQMessagesViewController.git`, branch `signal-master`)
+  - JSQMessagesViewController (from `../JSQMessagesViewController`)
   - OpenSSL (from `https://github.com/WhisperSystems/OpenSSL-Pod`)
   - PureLayout
   - Reachability
@@ -125,8 +125,7 @@ EXTERNAL SOURCES:
   AxolotlKit:
     :git: https://github.com/WhisperSystems/SignalProtocolKit.git
   JSQMessagesViewController:
-    :branch: signal-master
-    :git: https://github.com/WhisperSystems/JSQMessagesViewController.git
+    :path: ../JSQMessagesViewController
   OpenSSL:
     :git: https://github.com/WhisperSystems/OpenSSL-Pod
   SignalServiceKit:
@@ -138,9 +137,6 @@ CHECKOUT OPTIONS:
   AxolotlKit:
     :commit: 6e9d5e8c3dc87e397b4b7485172507b0990a22dd
     :git: https://github.com/WhisperSystems/SignalProtocolKit.git
-  JSQMessagesViewController:
-    :commit: 868cf70fae0f3b54c3cfa7773ef03578d209b668
-    :git: https://github.com/WhisperSystems/JSQMessagesViewController.git
   OpenSSL:
     :commit: b2d3c149102032a09aefbfb470885db4aa83efad
     :git: https://github.com/WhisperSystems/OpenSSL-Pod
@@ -171,6 +167,6 @@ SPEC CHECKSUMS:
   UnionFind: c33be5adb12983981d6e827ea94fc7f9e370f52d
   YapDatabase: cd911121580ff16675f65ad742a9eb0ab4d9e266
 
-PODFILE CHECKSUM: 2f847bb25e70d1d376f38cf21ae08624fa6ed67d
+PODFILE CHECKSUM: 4a6db311736b5ab36b660af813dcdf0af62da48c
 
 COCOAPODS: 1.2.1

Signal/src/ViewControllers/ConversationView/MessagesViewController.m

@@ -169,7 +169,8 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
     UIDocumentPickerDelegate,
     UIImagePickerControllerDelegate,
     UINavigationControllerDelegate,
-    UITextViewDelegate>
+    UITextViewDelegate,
+    JSQLayoutDelegate>
 
 @property (nonatomic) TSThread *thread;
 @property (nonatomic) TSMessageAdapter *lastDeliveredMessage;
@@ -245,6 +246,7 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
 @property (nonatomic) BOOL shouldObserveDBModifications;
 @property (nonatomic) BOOL viewHasEverAppeared;
 @property (nonatomic) BOOL wasScrolledToBottomBeforeKeyboardShow;
+@property (nonatomic) BOOL wasScrolledToBottomBeforeLayoutChange;
 
 @end
 
@@ -404,7 +406,7 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
     // We only want to update scroll state for non-empty show events.
     BOOL isPresenting = endValue.size.height > 0.f;
 
-    if (isPresenting) {
+    if (isPresenting && self.wasScrolledToBottomBeforeKeyboardShow) {
         [self scrollToBottomImmediately];
     }
 }
@@ -543,6 +545,8 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
 
 - (void)viewDidLoad
 {
+    self.collectionView.layoutDelegate = self;
+
     [super viewDidLoad];
 
     [self.navigationController.navigationBar setTranslucent:NO];
@@ -4461,6 +4465,24 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
     [self updateNavigationBarSubtitleLabel];
 }
 
+#pragma mark - JSQLayoutDelegate
+
+- (void)jsqWillChangeLayout
+{
+    OWSAssert([NSThread isMainThread]);
+
+    self.wasScrolledToBottomBeforeLayoutChange = [self isScrolledToBottom];
+}
+
+- (void)jsqDidChangeLayout
+{
+    OWSAssert([NSThread isMainThread]);
+
+    if (self.wasScrolledToBottomBeforeLayoutChange) {
+        [self scrollToBottomImmediately];
+    }
+}
+
 #pragma mark - Class methods
 
 + (UINib *)nib