Fix line geometry: clamp fake-round join anchor on short

GitOrigin-RevId: b456fd5b872ccd24c857a06c21e658893910311f

Author: mapbox-github-ci-writer-1[bot]

src/data/bucket/line_bucket.ts

@@ -877,9 +877,13 @@ class LineBucket implements Bucket {
                     this.addCurrentVertex(currentVertex, endNormal ? endNormal : prevNormal, -capScale, -capScale, segment, lineProgressFeatures);
                 }
 
-                const skipStraightEdges = dist <= 2 * sharpCornerOffset && currentJoin !== 'bevel';
+                // On short segments also pull the fake-round fan anchor from the miter point back to
+                // the unit circle: scaled by the line width, the miter point can land outside the
+                // geometry and its fan paints border color across neighbouring segments.
+                const skipStraightEdges = dist <= 1.0 * sharpCornerOffset && currentJoin !== 'bevel';
+                const joinExtent = skipStraightEdges ? 1.0 : miterLength;
                 const join = joinNormal.mult(lineTurnsLeft ? 1.0 : -1.0);
-                join._mult(miterLength);
+                join._mult(joinExtent);
                 const next = nextNormal.mult(lineTurnsLeft ? -1.0 : 1.0);
                 const prev = prevNormal.mult(lineTurnsLeft ? -1.0 : 1.0);
                 const lpf = this.evaluateLineProgressFeatures(this.distance);

test/integration/render-tests/line-border/gradient/expected.png

@@ -0,0 +1,35 @@
+{
+    "network": [
+        [
+            "probeNetwork - default - end",
+            0,
+            0
+        ],
+        [
+            "probeNetwork - default - start",
+            0,
+            0
+        ]
+    ],
+    "gfx": [
+        [
+            "probeGFX - default - end",
+            12,
+            2,
+            2,
+            1,
+            [
+                16388,
+                16388
+            ],
+            [
+                1404,
+                1404
+            ],
+            [
+                2816,
+                2816
+            ]
+        ]
+    ]
+}

test/integration/render-tests/line-border/short-segments-w-shape/expected.png

@@ -0,0 +1,35 @@
+{
+    "network": [
+        [
+            "probeNetwork - default - end",
+            0,
+            0
+        ],
+        [
+            "probeNetwork - default - start",
+            0,
+            0
+        ]
+    ],
+    "gfx": [
+        [
+            "probeGFX - default - end",
+            12,
+            0,
+            0,
+            0,
+            [
+                0,
+                0
+            ],
+            [
+                0,
+                0
+            ],
+            [
+                0,
+                0
+            ]
+        ]
+    ]
+}

test/integration/render-tests/line-border/short-segments-w-shape/metrics-native-linux-gl.json

@@ -0,0 +1,76 @@
+{
+  "version": 8,
+  "metadata": {
+    "test": {
+      "description": "Ensures that short segments in a wide line do not cause unclamped fake-round join anchors to extend beyond the geometry.",
+      "width": 512,
+      "height": 512,
+      "image-threshold": [
+        {
+          "platform-tag-contains": "",
+          "threshold": 1e-06
+        }
+      ]
+    }
+  },
+  "center": [0, 0],
+  "zoom": 14,
+  "bearing": 0,
+  "pitch": 0,
+  "sources": {
+    "single-s": {
+      "type": "geojson",
+      "tolerance": 0,
+      "data": {
+        "type": "Feature",
+        "properties": {},
+        "geometry": {
+          "type": "LineString",
+          "coordinates": [
+            [-0.005579, 0.000144],
+            [-0.000461, 0.000144],
+            [-0.000277, -0.000144],
+            [-0.000092, 0.000144],
+            [0.000092, -0.000144],
+            [0.000277, 0.000144],
+            [0.000461, -0.000144],
+            [0.005579, -0.000144]
+          ]
+        }
+      }
+    }
+  },
+  "layers": [
+    {
+      "id": "background",
+      "type": "background",
+      "paint": {
+        "background-color": "white"
+      }
+    },
+    {
+      "id": "line",
+      "type": "line",
+      "source": "single-s",
+      "layout": {
+        "line-cap": "round",
+        "line-join": "round"
+      },
+      "paint": {
+        "line-color": "rgb(60,110,180)",
+        "line-width": 44.0,
+        "line-border-color": "rgb(220,70,40)",
+        "line-border-width": 6.0
+      }
+    },
+    {
+      "id": "vertices",
+      "type": "circle",
+      "source": "single-s",
+      "paint": {
+        "circle-radius": 2.8,
+        "circle-color": "black"
+      }
+    }
+  ]
+}