Use half copper thickness as the thickness of the start and end of vias

Fixes https://gitlab.com/kicad/code/kicad/-/issues/8384

pcbnew/board_stackup_manager/board_stackup.cpp

@@ -712,6 +712,7 @@ int BOARD_STACKUP::GetLayerDistance( PCB_LAYER_ID aFirstLayer, PCB_LAYER_ID aSec
 
     int total = 0;
     bool start = false;
+    bool half  = false;
 
     for( BOARD_STACKUP_ITEM* item : m_list )
     {
@@ -725,17 +726,25 @@ int BOARD_STACKUP::GetLayerDistance( PCB_LAYER_ID aFirstLayer, PCB_LAYER_ID aSec
         if( !start && ( layer != UNDEFINED_LAYER && layer >= aFirstLayer ) )
         {
             start = true;
-            continue;
+            half = true;
         }
         else if( !start )
             continue;
 
         // Reached the stop copper layer?  we're done
         if( start && ( layer != UNDEFINED_LAYER && layer >= aSecondLayer ) )
-            break;
+            half = true;
 
         for( int sublayer = 0; sublayer < item->GetSublayersCount(); sublayer++ )
-            total += item->GetThickness( sublayer );
+        {
+            int subThickness = item->GetThickness( sublayer );
+            total += half ? ( subThickness / 2 ) : subThickness;
+        }
+
+        half = false;
+
+        if( layer != UNDEFINED_LAYER && layer >= aSecondLayer )
+            break;
     }
 
     return total;

pcbnew/board_stackup_manager/board_stackup.h

@@ -277,7 +277,12 @@ public:
      * Calculate the distance (height) between the two given copper layers.
      *
      * This factors in the thickness of any dielectric and copper layers between the two given
-     * layers, but not the height of the given copper layers.
+     * layers, and half the height of the given start and end layers.  This half-height calculation
+     * allows this to be used for consistent length measurements when calculating net length through
+     * a series of vias.  A more advanced algorithm would be possible once we have a good concept of
+     * the start and end for a length measurment, but for now this will do.
+     * See https://gitlab.com/kicad/code/kicad/-/issues/8384 for more background.
+     *
      * @param aFirstLayer is a copper layer
      * @param aSecondLayer is a different copper layer
      * @return the height (in IU) between the two layers