[core] always commit placement to fix #11795

Since placements will be committed even if they do not need the full
fade duration to fade features in, we need the new `fadeStartTime` to
keep track of how long we still need to fade. This is important because
if we fade too long we will trigger another placement and never stop
rendering.

src/mbgl/renderer/renderer_impl.cpp

@@ -387,6 +387,8 @@ void Renderer::Impl::render(const UpdateParameters& updateParameters) {
 
     bool placementChanged = false;
     if (!placement->stillRecent(parameters.timePoint)) {
+        placementChanged = true;
+
         auto newPlacement = std::make_unique<Placement>(parameters.state, parameters.mapMode);
         std::set<std::string> usedSymbolLayers;
         for (auto it = order.rbegin(); it != order.rend(); ++it) {
@@ -396,13 +398,9 @@ void Renderer::Impl::render(const UpdateParameters& updateParameters) {
             }
         }
 
-        placementChanged = newPlacement->commit(*placement, parameters.timePoint);
+        newPlacement->commit(*placement, parameters.timePoint);
         crossTileSymbolIndex.pruneUnusedLayers(usedSymbolLayers);
-        if (placementChanged || symbolBucketsChanged) {
-            placement = std::move(newPlacement);
-        }
-
-        placement->setRecent(parameters.timePoint);
+        placement = std::move(newPlacement);
 
         updateFadingTiles();
     } else {

src/mbgl/text/placement.cpp

@@ -38,7 +38,6 @@ Placement::Placement(const TransformState& state_, MapMode mapMode_)
     : collisionIndex(state_)
     , state(state_)
     , mapMode(mapMode_)
-    , recentUntil(TimePoint::min())
 {}
 
 void Placement::placeLayer(RenderSymbolLayer& symbolLayer, const mat4& projMatrix, bool showCollisionBoxes) {
@@ -188,7 +187,7 @@ void Placement::placeLayerBucket(
     bucket.justReloaded = false;
 }
 
-bool Placement::commit(const Placement& prevPlacement, TimePoint now) {
+void Placement::commit(const Placement& prevPlacement, TimePoint now) {
     commitTime = now;
 
     bool placementChanged = false;
@@ -222,7 +221,7 @@ bool Placement::commit(const Placement& prevPlacement, TimePoint now) {
         }
     }
 
-    return placementChanged;
+    fadeStartTime = placementChanged ? commitTime : prevPlacement.fadeStartTime;
 }
 
 void Placement::updateLayerOpacities(RenderSymbolLayer& symbolLayer) {
@@ -339,18 +338,15 @@ float Placement::symbolFadeChange(TimePoint now) const {
 }
 
 bool Placement::hasTransitions(TimePoint now) const {
-    return symbolFadeChange(now) < 1.0 || stale;
+    if (mapMode == MapMode::Continuous) {
+        return stale || std::chrono::duration<float>(now - fadeStartTime) < Duration(std::chrono::milliseconds(300));
+    } else {
+        return false;
+    }
 }
 
 bool Placement::stillRecent(TimePoint now) const {
-    return mapMode == MapMode::Continuous && recentUntil > now;
-}
-void Placement::setRecent(TimePoint now) {
-    stale = false;
-    if (mapMode == MapMode::Continuous) {
-        // Only set in continuous mode because "now" isn't defined in still mode
-        recentUntil = now + Duration(std::chrono::milliseconds(300));
-    }
+    return mapMode == MapMode::Continuous && commitTime + Duration(std::chrono::milliseconds(300)) > now;
 }
 
 void Placement::setStale() {

src/mbgl/text/placement.hpp

@@ -63,7 +63,7 @@ class Placement {
 public:
     Placement(const TransformState&, MapMode mapMode);
     void placeLayer(RenderSymbolLayer&, const mat4&, bool showCollisionBoxes);
-    bool commit(const Placement& prevPlacement, TimePoint);
+    void commit(const Placement& prevPlacement, TimePoint);
     void updateLayerOpacities(RenderSymbolLayer&);
     float symbolFadeChange(TimePoint now) const;
     bool hasTransitions(TimePoint now) const;
@@ -94,12 +94,12 @@ class Placement {
 
     TransformState state;
     MapMode mapMode;
+    TimePoint fadeStartTime;
     TimePoint commitTime;
 
     std::unordered_map<uint32_t, JointPlacement> placements;
     std::unordered_map<uint32_t, JointOpacityState> opacities;
 
-    TimePoint recentUntil;
     bool stale = false;
 
     std::unordered_map<uint32_t, RetainedQueryData> retainedQueryData;