Fix scrollbars in Mojave dark mode not rendering properly

Dark mode scrollbars's background are rendered in a translucent color so
that it would overlay on top of the background nearly, even though we
are using legacy scrollbars with a dedicated space. This is unlike light
mode scrollbars which render with a concrete color. This means if the
background has uncleared rendering it would result in some oddities.
Just fix it by making sure we first render the scroll track with the
current background color before drawing the system overlay.

Known issues:
- The scroll knob will look quite light and hard to see if the
  background color is bright. Consider this OK for now as users who use
  dark mode will likely have a dark-ish background color in Vim.
- If both vertical and horizontal scrollbars are enabled, the corners
  will look black or sometimes filled with rendering artifacts. Will fix
  this later. One way to fix is to always fill the Vim view with a
  background color but that seems to slow things down a little bit
  because setNeedsDisplay seems to be called too much without a rect, so
  need to fix that first.
- Vertical scrollbars' positions are sometimes set incorrectly. That's a
  separate bug and will be addressed later.
- MacVim currently doesn't support overlay scrollbars which have been
  introduced in macOS since 10.7. Investigate that option too.

Author: ychin

src/MacVim/MMVimView.m

@@ -494,6 +494,11 @@ - (void)setScrollbarPosition:(int)pos length:(int)len identifier:(int32_t)ident
 - (void)setDefaultColorsBackground:(NSColor *)back foreground:(NSColor *)fore
 {
     [textView setDefaultColorsBackground:back foreground:fore];
+
+    for (NSUInteger i = 0, count = [scrollbars count]; i < count; ++i) {
+        MMScroller *sb = [scrollbars objectAtIndex:i];
+        [sb setNeedsDisplay:YES];
+    }
 }
 
 
@@ -801,10 +806,15 @@ - (void)placeScrollbars
         }
     }
 
-    // HACK: If there is no bottom or right scrollbar the resize indicator will
-    // cover the bottom-right corner of the text view so tell NSWindow not to
-    // draw it in this situation.
-    [[self window] setShowsResizeIndicator:(rightSbVisible||botSbVisible)];
+    if (NSAppKitVersionNumber < NSAppKitVersionNumber10_7) {
+        // HACK: If there is no bottom or right scrollbar the resize indicator will
+        // cover the bottom-right corner of the text view so tell NSWindow not to
+        // draw it in this situation.
+        //
+        // Note: This API is ignored from 10.7 onward and is now deprecated. This
+        // should be removed if we want to drop support for 10.6.
+        [[self window] setShowsResizeIndicator:(rightSbVisible||botSbVisible)];
+    }
 }
 
 - (NSUInteger)representedIndexOfTabViewItem:(NSTabViewItem *)tvi
@@ -973,6 +983,22 @@ - (id)initWithIdentifier:(int32_t)ident type:(int)theType
     return self;
 }
 
+- (void)drawKnobSlotInRect:(NSRect)slotRect highlight:(BOOL)flag
+{
+    // Dark mode scrollbars draw a translucent knob slot overlaid on top of
+    // whatever background the view has, even when we are using legacy
+    // scrollbars with a dedicated space.  This means we need to draw the
+    // background with some colors first, or else it would look really black, or
+    // show through rendering artifacts (e.g. if guioption 'k' is on, and you
+    // turn off the bar bar, the artiacts will show through in the overlay).
+    MMVimView *vimView = [self target];
+    NSColor *defaultBackgroundColor = [[vimView textView] defaultBackgroundColor];
+    [defaultBackgroundColor setFill];
+    NSRectFill(slotRect);
+
+    [super drawKnobSlotInRect:slotRect highlight:flag];
+}
+
 - (int32_t)scrollerId
 {
     return identifier;