Mark pinned child windows as fullscreen-auxiliary

Adding a child window without NSWindowCollectionBehaviorFullScreenAuxiliary
to a fullscreen parent forces AppKit to demote the parent out of
fullscreen so the two can coexist on the regular desktop. Result:
opening Preferences from fullscreen kicked the emulator out of
fullscreen rather than overlaying Preferences on the fullscreen
Space.

OR the auxiliary flag into the child's collection behavior in
pinAsChildOf:. The bit is harmless when the parent is windowed and
gives every utility window the right Space-sharing semantics for
free, since they all reach the fullscreen state through this single
choke point.

Author: morozov

fusepb/views/DisplayOpenGLView.m

@@ -1433,6 +1433,13 @@ @implementation NSWindow (FuseChildPinning)
 - (void)pinAsChildOf:(NSWindow *)parent
 {
   if( !parent || parent == self ) return;
+  /* Without FullScreenAuxiliary, adding self as a child of a fullscreen
+     parent forces AppKit to exit fullscreen so the two can coexist on
+     the regular desktop. With it, both windows share the parent's
+     fullscreen Space. Harmless when the parent is windowed. */
+  [self setCollectionBehavior:
+    [self collectionBehavior] |
+    NSWindowCollectionBehaviorFullScreenAuxiliary];
   NSWindow *currentParent = [self parentWindow];
   if( currentParent == parent ) return;
   if( currentParent ) {