guard against redoing backfill work for the borderTile

1 files changed, 7 insertions, 1 deletions

diff --git a/src/mbgl/renderer/sources/render_raster_dem_source.cpp b/src/mbgl/renderer/sources/render_raster_dem_source.cpp
index b8bb2ce1cd..a91815563f 100644
--- a/src/mbgl/renderer/sources/render_raster_dem_source.cpp
+++ b/src/mbgl/renderer/sources/render_raster_dem_source.cpp
@@ -114,7 +114,13 @@ void RenderRasterDEMSource::onTileChanged(Tile& tile){
                 if (renderableNeighbor != nullptr && renderableNeighbor->isRenderable()) {
                     RasterDEMTile& borderTile = static_cast<RasterDEMTile&>(*renderableNeighbor);
                     demtile.backfillBorder(borderTile, mask);
-                    borderTile.backfillBorder(demtile, opposites[mask]);
+
+                    // if the border tile has not been backfilled by a previous instance of the main
+                    // tile, backfill its corresponding neighbor as well.
+                    const DEMTileNeighbors& borderMask = opposites[mask];
+                    if ((borderTile.neighboringTiles & borderMask) != borderMask){
+                        borderTile.backfillBorder(demtile, borderMask);
+                     }
                 }
             }
         }