fix pec frame lengths

Author: dbochkov-flexcompute

tidy3d/components/simulation.py

@@ -5646,16 +5646,12 @@ def _make_pec_frame(self, obj: Union[ModeSource, InternalAbsorber]) -> Structure
         if isinstance(obj, ModeSource):
             axis = obj.injection_axis
             length = obj.frame.length
+            if direction == "+":
+                span_inds[axis][1] += length - 1
+            else:
+                span_inds[axis][0] -= length - 1
         else:
             axis = obj.size.index(0.0)
-            length = 1
-
-        if direction == "+":
-            span_inds[axis][1] += length - 1
-            span_inds[axis][0] -= 1
-        else:
-            span_inds[axis][1] += 1
-            span_inds[axis][0] -= length - 1
 
         box_bounds = [
             [