[core] Place already visible symbols first and hidden symbols last

src/mbgl/text/placement.cpp

@@ -662,7 +662,25 @@ void Placement::placeSymbolBucket(const BucketPlacementData& params, std::set<ui
         }
 
     } else {
-        for (const SymbolInstance& symbol : bucket.getSymbols(params.sortKeyRange)) {
+        auto sortedSymbols = bucket.getSymbols(params.sortKeyRange);
+        if (auto* previousPlacement = getPrevPlacement()) {
+            std::stable_sort(
+                sortedSymbols.begin(), sortedSymbols.end(), [&](const SymbolInstance& a, const SymbolInstance& b) {
+                    auto* aPlacement = previousPlacement->getSymbolPlacement(a);
+                    auto* bPlacement = previousPlacement->getSymbolPlacement(b);
+                    if (!aPlacement) {
+                        // a < b, if 'a' is new and if 'b' was previously hidden.
+                        return bPlacement && !bPlacement->placed();
+                    }
+                    if (!bPlacement) {
+                        // a < b, if 'b' is new and 'a' was previously shown.
+                        return aPlacement && aPlacement->placed();
+                    }
+                    // a < b, if 'a' was shown and 'b' was hidden.
+                    return aPlacement->placed() && !bPlacement->placed();
+                });
+        }
+        for (const SymbolInstance& symbol : sortedSymbols) {
             placeSymbol(symbol);
         }
     }
@@ -1203,6 +1221,13 @@ float Placement::zoomAdjustment(const float zoom) const {
     return std::max(0.0, (placementZoom - zoom) / 1.5);
 }
 
+const JointPlacement* Placement::getSymbolPlacement(const SymbolInstance& symbol) const {
+    assert(symbol.crossTileID != 0);
+    auto found = placements.find(symbol.crossTileID);
+    if (found == placements.end()) return nullptr;
+    return &found->second;
+}
+
 Duration Placement::getUpdatePeriod(const float zoom) const {
     // Even if transitionOptions.duration is set to a value < 300ms, we still wait for this default transition duration
     // before attempting another placement operation.

src/mbgl/text/placement.hpp

@@ -49,6 +49,8 @@ public:
         : text(text_), icon(icon_), skipFade(skipFade_)
     {}
 
+    bool placed() const { return text || icon; }
+
     const bool text;
     const bool icon;
     // skipFade = outside viewport, but within CollisionIndex::viewportPadding px of the edge
@@ -119,6 +121,7 @@ public:
     Duration getUpdatePeriod(const float zoom) const;
 
     float zoomAdjustment(const float zoom) const;
+    const JointPlacement* getSymbolPlacement(const SymbolInstance&) const;
 
     const RetainedQueryData& getQueryData(uint32_t bucketInstanceId) const;