[core] Hold on to tile yStretch value for rendering old symbolBuckets while waiting for new ones.

src/mbgl/tile/geometry_tile.cpp

@@ -42,7 +42,8 @@ GeometryTile::GeometryTile(const OverscaledTileID& id_,
              parameters.pixelRatio),
       glyphManager(parameters.glyphManager),
       imageManager(parameters.imageManager),
-      placementThrottler(Milliseconds(300), [this] { invokePlacement(); }) {
+      placementThrottler(Milliseconds(300), [this] { invokePlacement(); }),
+      lastYStretch(1.0f) {
 }
 
 GeometryTile::~GeometryTile() {
@@ -143,6 +144,9 @@ void GeometryTile::onPlacement(PlacementResult result) {
     if (result.iconAtlasImage) {
         iconAtlasImage = std::move(*result.iconAtlasImage);
     }
+    if (collisionTile.get()) {
+        lastYStretch = collisionTile->yStretch;
+    }
     observer->onTileChanged(*this);
 }
 
@@ -265,7 +269,10 @@ void GeometryTile::querySourceFeatures(
 }
 
 float GeometryTile::yStretch() const {
-    return collisionTile->yStretch;
+    // collisionTile gets reset in onLayout but we don't clear the symbolBuckets
+    // until a new placement result comes along, so keep the yStretch value in
+    // case we need to render them.
+    return lastYStretch;
 }
 
 } // namespace mbgl

src/mbgl/tile/geometry_tile.hpp

@@ -122,6 +122,7 @@ private:
     std::unique_ptr<CollisionTile> collisionTile;
     
     util::Throttler placementThrottler;
+    float lastYStretch;
 
 public:
     optional<gl::Texture> glyphAtlasTexture;