Cleanup: clarify the purpose of full-screen areas

It wasn't obvious why a full-screen area should also disable gizmos.
Also move screen check last to avoid context member access.

source/blender/editors/space_view3d/view3d_gizmo_navigate.cc

@@ -153,8 +153,8 @@ static bool WIDGETGROUP_navigate_poll(const bContext *C, wmGizmoGroupType * /*gz
   View3D *v3d = CTX_wm_view3d(C);
   if ((((U.uiflag & USER_SHOW_GIZMO_NAVIGATE) == 0) &&
        (U.mini_axis_type != USER_MINI_AXIS_TYPE_GIZMO)) ||
-      (CTX_wm_screen(C)->state == SCREENFULL) ||
-      (v3d->gizmo_flag & (V3D_GIZMO_HIDE | V3D_GIZMO_HIDE_NAVIGATE)))
+      (v3d->gizmo_flag & (V3D_GIZMO_HIDE | V3D_GIZMO_HIDE_NAVIGATE)) ||
+      (CTX_wm_screen(C)->state == SCREENFULL))
   {
     return false;
   }

source/blender/makesdna/DNA_screen_types.h

@@ -591,7 +591,12 @@ enum {
   SCREENNORMAL = 0,
   /** One editor taking over the screen. */
   SCREENMAXIMIZED = 1,
-  /** One editor taking over the screen with no bare-minimum UI elements. */
+  /**
+   * One editor taking over the screen with no bare-minimum UI elements.
+   *
+   * Besides making the area full-screen this disables navigation & statistics because
+   * this is part of a stereo 3D pipeline where these elements would interfere, see: !142418.
+   */
   SCREENFULL = 2,
 };