Set root view to be opaque

The idea was that e.g. a status bar may wish to not be opaque and rely on the window’s border to shine through.

Unfortunately there is an issue where if the view is not opaque, the user can drag the entire window by clicking the scroll knobs (requires scrollbars to be configured to be visible and using a mouse, a trackpad doesn’t seem to cause the issue).

Frameworks/DocumentWindow/src/ProjectLayoutView.mm

@@ -320,6 +320,11 @@ NSString* const kUserDefaultsHTMLOutputSizeKey   = @"htmlOutputSize";
 	[self addCursorRect:[self htmlOutputResizeRect]  cursor:[NSCursor resizeUpDownCursor]];
 }
 
+- (BOOL)isOpaque
+{
+	return YES;
+}
+
 - (NSView*)hitTest:(NSPoint)aPoint
 {
 	if(NSMouseInRect([self convertPoint:aPoint fromView:[self superview]], [self fileBrowserResizeRect], [self isFlipped]))