[core] Render sources keep the existing tiles until the new tileset is ready

In Continuous map mode, keep the existing tiles if the new tileset is not
yet available, thus providing smart style transitions without flickering.

1 files changed, 8 insertions, 4 deletions

diff --git a/src/mbgl/renderer/sources/render_raster_dem_source.cpp b/src/mbgl/renderer/sources/render_raster_dem_source.cpp
index 841124fa4a..06ddf3e7e0 100644
--- a/src/mbgl/renderer/sources/render_raster_dem_source.cpp
+++ b/src/mbgl/renderer/sources/render_raster_dem_source.cpp
@@ -4,6 +4,7 @@
 #include <mbgl/algorithm/update_tile_masks.hpp>
 #include <mbgl/geometry/dem_data.hpp>
 #include <mbgl/renderer/buckets/hillshade_bucket.hpp>
+#include <mbgl/renderer/tile_parameters.hpp>
 
 namespace mbgl {
 
@@ -27,16 +28,19 @@ void RenderRasterDEMSource::update(Immutable<style::Source::Impl> baseImpl_,
     enabled = needsRendering;
 
     optional<Tileset> _tileset = impl().getTileset();
-
-    if (tileset != _tileset) {
+    // In Continuous mode, keep the existing tiles if the new tileset is not
+    // yet available, thus providing smart style transitions without flickering.
+    // In other modes, allow clearing the tile pyramid first, before the early
+    // return in order to avoid render tests being flaky.
+    bool canUpdateTileset = _tileset || parameters.mode != MapMode::Continuous;
+    if (canUpdateTileset && tileset != _tileset) {
         tileset = _tileset;
         maxzoom = tileset->zoomRange.max;
         // TODO: this removes existing buckets, and will cause flickering.
         // Should instead refresh tile data in place.
         tilePyramid.clearAll();
     }
-    // Allow clearing the tile pyramid first, before the early return in case
-    //  the new tileset is not yet available or has an error in loading
+
     if (!_tileset) {
         return;
     }