tweak logic so manual is always in same position as "OneSided"

jdaviz/configs/specviz2d/plugins/spectral_extraction/spectral_extraction.py

@@ -320,20 +320,22 @@ def _trace_dataset_selected(self, msg=None):
 
         # sign for one-sided and single trace-pixel depending on whether the brightest pixel is
         # above or below the middle of the image
-        sign = 1 if (brightest_pixel < width / 2) else -1
+        if default_bg_width * 2 >= distance_from_edge:
+            sign = 1 if (brightest_pixel < width / 2) else -1
+            default_bg_separation = sign * default_bg_width * 2
+        else:
+            default_bg_separation = default_bg_width * 2
 
         if self.trace_pixel == 0:
             self.trace_pixel = brightest_pixel
         if self.trace_window == 0:
             self.trace_window = default_width
         if self.bg_trace_pixel == 0:
-            self.bg_trace_pixel = brightest_pixel + sign * default_bg_width * 2
+            self.bg_trace_pixel = brightest_pixel + default_bg_separation
         if self.bg_separation == 0:
             if default_bg_width * 2 >= distance_from_edge:
                 self.bg_type_selected = 'OneSided'
-                self.bg_separation = sign * default_bg_width * 2
-            else:
-                self.bg_separation = default_bg_width * 2
+            self.bg_separation = default_bg_separation
         if self.bg_width == 0:
             self.bg_width = default_bg_width
         if self.ext_width == 0: