fix: line-layer-opacity clipping subsequent same-source layers (#7867)

* test: add regression test for line-layer-opacity clipping a subsequent same-source layer (#7865)

Adds a render test reproducing #7865: a shared GeoJSON source with three
filtered line layers where the middle layer uses line-layer-opacity. The
subsequent same-source layer disappears because the layer-opacity path
re-renders tile clipping masks into the scratch FBO and leaves
currentStencilSource pointing at the source, so the next layer is clipped
by stale stencil state.

expected.png is blessed here against the current (buggy) rendering — the
blue line is missing. The follow-up commit enables the fix and re-blesses,
so the restored line shows up as an image diff in this PR.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix: reset stencil-source after line/fill-layer-opacity composite (#7865)

Reset painter.currentStencilSource after compositing a layer-opacity layer
so a subsequent layer sharing the same source redraws its tile clipping
masks into the composite target instead of being clipped by stale stencil
state. Covers both line-layer-opacity and fill-layer-opacity.

Re-blesses the render test: the previously-missing line is now rendered.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: CommanderStorm

CHANGELOG.md

@@ -4,6 +4,7 @@
 - _...Add new stuff here..._
 
 ### 🐞 Bug fixes
+- Fix `line-layer-opacity`/`fill-layer-opacity` clipping away a subsequent layer that shares the same source ([#7867](https://github.com/maplibre/maplibre-gl-js/pull/7867)) (by [@CommanderStorm](https://github.com/CommanderStorm))
 - Fix camera jump in flyTo when minZoom is set ([#7743](https://github.com/maplibre/maplibre-gl-js/pull/7743)) (by [@YuChunTsao](https://github.com/YuChunTsao))
 - _...Add new stuff here..._
 

src/webgl/draw/draw_layer_opacity.ts

@@ -87,4 +87,8 @@ export function drawLayerOpacity(painter: Painter, opacity: number, prepareDrawL
         layerOpacityUniformValues(opacity, 0), null, null,
         layer.id, painter.viewportBuffer, painter.quadTriangleIndexBuffer,
         painter.viewportSegments, layer.paint, painter.transform.zoom);
+
+    // Clipping masks were drawn into the scratch FBO's stencil buffer, not the composite target's.
+    // Reset currentStencilSource so a later layer on the same source redraws its masks into the composite target instead of reusing stale ones.
+    painter.currentStencilSource = undefined;
 }

test/integration/render/tests/line-layer-opacity/shared-source-subsequent-layer/expected.png

@@ -0,0 +1,81 @@
+{
+  "version": 8,
+  "metadata": {
+    "test": {
+      "width": 64,
+      "height": 64
+    }
+  },
+  "sources": {
+    "geojson": {
+      "type": "geojson",
+      "data": {
+        "type": "FeatureCollection",
+        "features": [
+          {
+            "type": "Feature",
+            "properties": { "id": "red" },
+            "geometry": {
+              "type": "LineString",
+              "coordinates": [[-15, -15], [15, 15]]
+            }
+          },
+          {
+            "type": "Feature",
+            "properties": { "id": "green" },
+            "geometry": {
+              "type": "LineString",
+              "coordinates": [[15, -15], [-15, 15]]
+            }
+          },
+          {
+            "type": "Feature",
+            "properties": { "id": "blue" },
+            "geometry": {
+              "type": "LineString",
+              "coordinates": [[-15, 0], [15, 0]]
+            }
+          }
+        ]
+      }
+    }
+  },
+  "layers": [
+    {
+      "id": "background",
+      "type": "background",
+      "paint": { "background-color": "white" }
+    },
+    {
+      "id": "line-red",
+      "type": "line",
+      "source": "geojson",
+      "filter": ["==", ["get", "id"], "red"],
+      "paint": {
+        "line-width": 12,
+        "line-color": "#ff0000"
+      }
+    },
+    {
+      "id": "line-green",
+      "type": "line",
+      "source": "geojson",
+      "filter": ["==", ["get", "id"], "green"],
+      "paint": {
+        "line-width": 12,
+        "line-color": "#00ff00",
+        "line-layer-opacity": 0.5
+      }
+    },
+    {
+      "id": "line-blue",
+      "type": "line",
+      "source": "geojson",
+      "filter": ["==", ["get", "id"], "blue"],
+      "paint": {
+        "line-width": 12,
+        "line-color": "#0000ff"
+      }
+    }
+  ]
+}