When you want to keep the scroll position right when scroll view resizes.

UIScrollView+AdjustOffset.m

@interface UIScrollView (AdjustOffset)

- (CGFloat)updateContentOffsetWithPreviousHeight:(CGFloat)previousHeight distanceFromBottom:(CGFloat)distanceFromBottom;

@end

@implementation UIScrollView (AdjustOffset)

- (void)updateContentOffset:(CGPoint)offset
{
    self.contentOffset = [self adjustContentOffset:offset];
}

- (CGPoint )adjustContentOffset:(CGPoint)offset
{
    if (offset.y < 0) {
        offset.y = 0;
        return offset;
    }
    else if (offset.y > self.contentSize.height - self.frame.size.height) {
        offset.y = self.contentSize.height - self.frame.size.height;

        return offset;
    }

    return offset;
}

- (CGFloat)offsetYWhenHeightResizedBiggerWithDistanceFromBottom:(CGFloat)distanceFromBottom
{
    CGFloat hiddenContentHeight = self.contentSize.height - self.frame.size.height;

    return hiddenContentHeight - distanceFromBottom;
}

- (CGFloat)distanceFromBttomWithHeight:(CGFloat)height
{
    return self.contentSize.height - height - self.contentOffset.y;
}

- (CGFloat)updateContentOffsetWithPreviousHeight:(CGFloat)previousHeight distanceFromBottom:(CGFloat)distanceFromBottom
{
    CGPoint offset =  self.contentOffset;
    CGFloat diff = previousHeight - self.frame.size.height;

    // update distanceFromBottom
    if (diff >= 0) {
        distanceFromBottom = [self distanceFromBttomWithHeight:previousHeight];
    }

    [self updateContentOffset:({
        if (diff < 0) { // getting bigger
            offset.y = [self offsetYWhenHeightResizedBiggerWithDistanceFromBottom:distanceFromBottom];
        } else { // getting smaller
            offset.y += diff;
        }
        offset;
    })];
    
    return distanceFromBottom;
}

@end