Ensure onboarding views never reclaim layout space from dismissed keyboard.

Signal/src/ViewControllers/Registration/OnboardingBaseViewController.swift

@@ -73,6 +73,12 @@ public class OnboardingBaseViewController: OWSViewController {
 
     // MARK: - View Lifecycle
 
+    public override func viewDidLoad() {
+        super.viewDidLoad()
+
+        self.shouldBottomViewReserveSpaceForKeyboard = true
+    }
+
     public override func viewWillAppear(_ animated: Bool) {
         super.viewWillAppear(animated)
 

SignalMessaging/ViewControllers/OWSViewController.h

@@ -20,6 +20,10 @@ UIInterfaceOrientationMask DefaultUIInterfaceOrientationMask(void);
 // BUT adjust its location upward if the keyboard appears.
 - (void)autoPinViewToBottomOfViewControllerOrKeyboard:(UIView *)view avoidNotch:(BOOL)avoidNotch;
 
+// If YES, the bottom view never "reclaims" layout space if the keyboard is dismissed.
+// Defaults to NO.
+@property (nonatomic) BOOL shouldBottomViewReserveSpaceForKeyboard;
+
 @end
 
 NS_ASSUME_NONNULL_END

SignalMessaging/ViewControllers/OWSViewController.m

@@ -211,7 +211,11 @@ UIInterfaceOrientationMask DefaultUIInterfaceOrientationMask(void)
         // There's no need to use: [UIView animateWithDuration:...].
         // Any layout changes made during these notifications are
         // automatically animated.
-        self.bottomLayoutConstraint.constant = offset;
+        if (self.shouldBottomViewReserveSpaceForKeyboard) {
+            self.bottomLayoutConstraint.constant = MIN(self.bottomLayoutConstraint.constant, offset);
+        } else {
+            self.bottomLayoutConstraint.constant = offset;
+        }
         [self.bottomLayoutView.superview layoutIfNeeded];
     };