NSWindow resize indicator not visible

I have not checked this, but you could set the resize indicators manually. I think I would add four NSTrackingAreas to the windows contentView subclass (one for each side of the window, only few pixels in height/width). In the mouseEntered() method, create a new NSCursor object for the appropriate mouse position. Remember that the position could change, so use the mouseMoved() method as well. On mouseExited() reset the cursor.

Again, I have not tried this, but it should work.

PS: Don't forget to file a radar on this ;)

I've fixed this issue by subclassing NSWindow and overriding canBecomeKeyWindow to return YES:

#import "MyWindow.h"

@implementation MyWindow

- (BOOL)canBecomeKeyWindow {
    return YES;
}

@end

Not updating resize cursors in this case looks like Apple bug. Documentation states "The value of canBecomeKeyWindow property is YES if the window has a title bar or a resize bar, or NO otherwise.", so I expect that canBecomeKeyWindow will return YES for resizable window. But it doesn't.

UPD: Checked on 10.10.5. Hopefully, you will have same behaviour on 10.11.