Fix scrollbar rendering artifact in pre-Mojave renderer

Fix scrollbars not rendering properly when using delayed calls to
placeScrollBars. That has issues since pre-Mojave renderer blocks the
window from changing size by using `[NSAnimationContext beginGrouping]`.
That causes the bug as placeScrollbars would be using state from
previous sizes.

We could fix this by waiting for the animation to end before called
finishPlaceScrollbars but it's more complicated and leads to more state
tracking. Instead, just call placeScrollbars when we resize immediately
(just like before) instead of deferring. We still defer placing
scrollbars for things like creating/deleting scrollbars as we could be
doing that a lot of times per frame (e.g. calling `:only` with a lot of
splits).

Fix #848

Author: ychin

src/MacVim/MMVimView.m

@@ -929,7 +929,19 @@ - (void)frameSizeMayHaveChanged:(BOOL)keepGUISize
     NSRect textViewRect = [self textViewRectForVimViewSize:[self frame].size];
     [textView setFrame:textViewRect];
 
-    self.pendingPlaceScrollbars = YES;
+    // Immediately place the scrollbars instead of deferring till later here.
+    // Deferral ended up causing some bugs, in particular when in <10.14
+    // CoreText renderer where [NSAnimationContext beginGrouping] is used to
+    // bundle state changes together and the deferred placeScrollbars would get
+    // the wrong data to use. An alternative would be to check for that and only
+    // call finishPlaceScrollbars once we call [NSAnimationContext endGrouping]
+    // but that makes the code mode complicated. Just do it here and the
+    // performance is fine as this gets called occasionally only
+    // (pendingPlaceScrollbars is mostly for the case if we are adding a lot of
+    // scrollbars at once we want to only call placeScrollbars once instead of
+    // doing it N times).
+    self.pendingPlaceScrollbars = NO;
+    [self placeScrollbars];
 
     // It is possible that the current number of (rows,columns) is too big or
     // too small to fit the new frame.  If so, notify Vim that the text