fix(render): release the atmosphere probe's WebGL context

Lose the throwaway WebGL2 probe context right after the link probe instead
of leaving it for GC; browsers (Safari especially) cap live contexts. Note
that affected devices also lose globe-basemap fog, credit the changelog
entry and move it to the top.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CHANGELOG.md

@@ -1,5 +1,19 @@
 # Changelog
 
+- Render on iPad and iPhone instead of stopping with "An error occurred while
+  rendering." Cesium's per-vertex model atmosphere binds shader `out`
+  parameters directly to varyings, which Apple's Metal/ANGLE backend cannot
+  link, so the program failed and the render loop was torn down. The stage is
+  now kept out of the pipeline on affected devices by clearing
+  `scene.fog.renderable`, which leaves `fog.enabled` — and the fog density that
+  drives 3D Tiles refinement — untouched. Detection is a WebGL2 link probe of
+  the same pattern, so a future driver fix restores the effect with no code
+  change, with iOS/iPadOS detection as a backstop. Sky atmosphere and the
+  ground-atmosphere fragment path route through locals and are unaffected;
+  affected devices lose distance fog on 3D tiles and on the globe basemaps.
+  `src/app/atmosphereCompat.test.mjs` pins the probe, the platform matrix and
+  the `renderable`-not-`enabled` choice (KnottyDyes, #705).
+
 - Region scopes in voice analyst queries ("in the Gulf of Mexico", "over
   the Alps") work again in the dev server: the bundled Natural Earth and
   neighborhood packs are fetched as JSON in the browser
@@ -196,19 +210,6 @@ Analyst records for loaded satellites, datacenters and dams, with explicit bound
   retain geometry; incident-link checks abort on disable or selection change,
   and the row includes a containment legend.
 
-- Render on iPad and iPhone instead of stopping with "An error occurred while
-  rendering." Cesium's per-vertex model atmosphere binds shader `out`
-  parameters directly to varyings, which Apple's Metal/ANGLE backend cannot
-  link, so the program failed and the render loop was torn down. The stage is
-  now kept out of the pipeline on affected devices by clearing
-  `scene.fog.renderable`, which leaves `fog.enabled` — and the fog density that
-  drives 3D Tiles refinement — untouched. Detection is a WebGL2 link probe of
-  the same pattern, so a future driver fix restores the effect with no code
-  change, with iOS/iPadOS detection as a backstop. Sky atmosphere and the
-  ground-atmosphere fragment path route through locals and are unaffected.
-  `src/app/atmosphereCompat.test.mjs` pins the probe, the platform matrix and
-  the `renderable`-not-`enabled` choice.
-
 - Remove the spurious scrollbars that appeared on both panel stacks at narrow
   widths (720px and below) as soon as a panel was expanded. The stacks scroll
   vertically there, and each panel's decorative glow, absolutely positioned

docs/CURRENT-STATE.md

@@ -701,7 +701,6 @@ snapshots, lifecycle actions and row descriptors. Remount and teardown remove
 listeners and row subscriptions; obsolete completions do not repaint old rows.
 The clear control presents busy state while its existing action owns the transaction.
 
-
 ## Model atmosphere on Apple Metal
 
 Cesium's per-vertex model atmosphere is kept out of the pipeline on devices
@@ -712,13 +711,15 @@ attaches that stage only when `fog.enabled && fog.renderable`, so the viewer
 clears `scene.fog.renderable` and leaves `fog.enabled` — and the fog density
 that drives 3D Tiles screen-space-error scaling — intact. Sky atmosphere and the
 ground-atmosphere fragment path route through locals and are unaffected.
+Affected devices lose distance fog on 3D tiles and on globe basemaps (Esri,
+Bing, keyless), since the globe fog shader uses the same `renderable` flag.
+The probe releases its throwaway WebGL2 context immediately.
 
 Detection is a WebGL2 link probe of the same out-parameter/varying pattern, so a
 future driver fix restores the effect with no code change; iOS/iPadOS platform
 detection is the backstop for when no probe context can be created. Applied
 during viewer construction, before any tile builds a draw command.
 
-
 ## Map Source control ownership
 
 Map Source controls own chip listeners, source-state subscriptions and selection

src/app/atmosphereCompat.js

@@ -154,7 +154,19 @@ export function shouldDisableModelAtmosphere({
 } = {}) {
   const gl = createProbeContext();
   if (gl) {
-    if (probeOutParamVaryingLinkFailure(gl)) return true;
+    let linkFails;
+    try {
+      linkFails = probeOutParamVaryingLinkFailure(gl);
+    } finally {
+      // Browsers cap live WebGL contexts (Safari especially); release the
+      // throwaway probe context now rather than waiting for GC.
+      try {
+        gl.getExtension?.('WEBGL_lose_context')?.loseContext();
+      } catch {
+        // A context that cannot be released is left to GC.
+      }
+    }
+    if (linkFails) return true;
     // Probe ran and the driver accepted the pattern. Apple mobile is still
     // quarantined: the probe program is far simpler than Cesium's, and a
     // false negative there costs the entire scene.

src/app/atmosphereCompat.test.mjs

@@ -207,3 +207,15 @@ test('tolerates a scene with no fog object', () => {
   assert.equal(applyModelAtmosphereWorkaround({}, {}), false);
   assert.equal(applyModelAtmosphereWorkaround(null, {}), false);
 });
+
+test('releases the throwaway probe context after probing', () => {
+  let lost = 0;
+  const gl = fakeGl({ linkOk: true });
+  gl.getExtension = (name) =>
+    name === 'WEBGL_lose_context' ? { loseContext: () => (lost += 1) } : null;
+  shouldDisableModelAtmosphere({
+    navigatorLike: LINUX_DESKTOP,
+    createProbeContext: () => gl,
+  });
+  assert.equal(lost, 1, 'probe context must not linger until GC');
+});