Reset user scroll when the buffer is cleared. Change the convenience accessor...

Reset user scroll when the buffer is cleared. Change the convenience accessor for verticalScroller to ptyVerticalScroller since in xcode 9 you can no longer override a superclass method and change the return type to a subclass of the method's original return type. Issue 6174

sources/MarkTrigger.m

@@ -77,7 +77,7 @@ typedef enum {
                                     stop:(BOOL *)stop {
     [aSession.screen terminalSaveScrollPositionWithArgument:@"saveCursorLine"];
     if ([self shouldStopScrolling]) {
-        [[aSession.view.scrollview verticalScroller] setUserScroll:YES];
+        [[aSession.view.scrollview ptyVerticalScroller] setUserScroll:YES];
     }
     return YES;
 }

sources/PTYScrollView.h

@@ -38,7 +38,7 @@
 @interface PTYScrollView : NSScrollView
 // More specific type for the base class's method.
-- (PTYScroller *)verticalScroller;
+- (PTYScroller *)ptyVerticalScroller;
 - (instancetype)initWithFrame:(NSRect)frame hasVerticalScroller:(BOOL)hasVerticalScroller;
 - (void)detectUserScroll;

sources/PTYScrollView.m

@@ -135,7 +135,7 @@
                [NSValue valueWithRect:[self documentVisibleRect]]];
 }
-- (PTYScroller *)verticalScroller {
+- (PTYScroller *)ptyVerticalScroller {
     return (PTYScroller *)[super verticalScroller];
 }

sources/PTYSession.m

@@ -1984,7 +1984,7 @@ ITERM_WEAKLY_REFERENCEABLE
             // beautiful here, but in that case we want to turn off the bell and scroll to the
             // bottom.
             [self setBell:NO];
-            PTYScroller *verticalScroller = [_view.scrollview verticalScroller];
+            PTYScroller *verticalScroller = [_view.scrollview ptyVerticalScroller];
             [verticalScroller setUserScroll:NO];
         }
         NSData *data = [string dataUsingEncoding:encoding allowLossyConversion:YES];
@@ -3912,6 +3912,7 @@ ITERM_WEAKLY_REFERENCEABLE
         _shell.size = size;
         _shell.size = _screen.size;
     }
+    _view.scrollview.ptyVerticalScroller.userScroll = NO;
 }
 - (void)clearScrollbackBuffer {