Fix issue #6919: bring gl-js collision handling closer to gl-native

- Don't place features that _have_ text but don't have collision boxes
- Create a single collision box even when the length of a label is less than half the box size

Test updates:
- Add debug collision circles to line-center to verify collision behavior
- Add regression test that exercises case where a line label is almost exactly the same length as its line

src/symbol/collision_feature.js

@@ -109,7 +109,7 @@ class CollisionFeature {
                            bucketIndex: number,
                            overscaling: number) {
         const step = boxSize / 2;
-        const nBoxes = Math.floor(labelLength / step);
+        const nBoxes = Math.floor(labelLength / step) || 1;
         // We calculate line collision circles out to 300% of what would normally be our
         // max size, to allow collision detection to work on labels that expand as
         // they move into the distance

src/symbol/placement.js

@@ -186,16 +186,16 @@ export class Placement {
 
         const partiallyEvaluatedTextSize = symbolSize.evaluateSizeForZoom(bucket.textSizeData, this.transform.zoom, symbolLayoutProperties.properties['text-size']);
 
-        const iconWithoutText = !bucket.hasTextData() || layout.get('text-optional');
-        const textWithoutIcon = !bucket.hasIconData() || layout.get('icon-optional');
+        const textOptional = layout.get('text-optional');
+        const iconOptional = layout.get('icon-optional');
 
         const collisionGroup = this.collisionGroups.get(bucket.sourceID);
 
         for (const symbolInstance of bucket.symbolInstances) {
             if (!seenCrossTileIDs[symbolInstance.crossTileID]) {
 
-                let placeText = symbolInstance.feature.text !== undefined;
-                let placeIcon = symbolInstance.feature.icon !== undefined;
+                let placeText = false;
+                let placeIcon = false;
                 let offscreen = true;
 
                 let placedGlyphBoxes = null;
@@ -256,6 +256,9 @@ export class Placement {
                     offscreen = offscreen && placedIconBoxes.offscreen;
                 }
 
+                const iconWithoutText = textOptional || (symbolInstance.numGlyphVertices === 0 && symbolInstance.numVerticalGlyphVertices === 0);
+                const textWithoutIcon = iconOptional || symbolInstance.numIconVertices === 0;
+
                 // Combine the scales for icons and text.
                 if (!iconWithoutText && !textWithoutIcon) {
                     placeIcon = placeText = placeIcon && placeText;

test/integration/render-tests/regressions/mapbox-gl-js#6919/style.json

@@ -0,0 +1,62 @@
+{
+  "version": 8,
+  "metadata": {
+    "test": {
+        "debug": true,
+        "collisionDebug": true,
+      "description": "'Carribean Sea' crosses a tile boundary, but we don't draw the tile boundary in the test because JS and Native render tile boundaries differently.",
+      "height": 256,
+      "width": 1024
+    }
+  },
+  "center": [
+    -73,
+    15
+  ],
+  "zoom": 4.5,
+  "sources": {
+    "mapbox": {
+      "type": "vector",
+      "maxzoom": 14,
+      "tiles": [
+        "local://tiles/mapbox.mapbox-streets-v7/{z}-{x}-{y}.mvt"
+      ]
+    }
+  },
+  "glyphs": "local://glyphs/{fontstack}/{range}.pbf",
+  "layers": [
+    {
+      "id": "background",
+      "type": "background",
+      "paint": {
+        "background-color": "white"
+      }
+    },
+    {
+      "id": "line-center",
+      "type": "symbol",
+      "source": "mapbox",
+      "source-layer": "marine_label",
+      "layout": {
+        "text-field": "{name_en}",
+        "symbol-placement": "line-center",
+        "text-allow-overlap": true,
+        "text-size": 50,
+        "text-letter-spacing": 0.5,
+        "text-font": [
+          "Open Sans Semibold",
+          "Arial Unicode MS Bold"
+        ]
+      }
+    },
+    {
+      "id": "line",
+      "type": "line",
+      "source": "mapbox",
+      "source-layer": "marine_label",
+      "paint": {
+        "line-width": 1
+      }
+    }
+  ]
+}

test/integration/render-tests/symbol-placement/line-center-buffer/style.json

@@ -39,7 +39,7 @@
         "text-field": "{name_en}",
         "symbol-placement": "line-center",
         "text-allow-overlap": true,
-        "text-size": 50,
+        "text-size": 38,
         "text-letter-spacing": 0.5,
         "text-font": [
           "Open Sans Semibold",

test/integration/render-tests/symbol-placement/line-center/style.json

@@ -2,6 +2,7 @@
   "version": 8,
   "metadata": {
     "test": {
+      "collisionDebug": true,
       "height": 512
     }
   },