fix(js): keep performance clock bounded

crates/obscura-js/js/bootstrap.js

@@ -5361,14 +5361,14 @@ globalThis.XMLSerializer = class XMLSerializer {
 globalThis.performance = globalThis.performance || {
   now: (function() {
     // Monotonically non-decreasing: return the wall-clock offset, but never a
-    // value below the last one. The previous version added an uncapped per-call
-    // sub-millisecond bump that could exceed 1.0, so when the millisecond then
-    // ticked over and the bump reset to 0 the result went backwards.
-    var _last = 0;
+    // value below the last one. Equal readings are allowed, and avoiding a
+    // synthetic per-call increment keeps tight loops from advancing the clock
+    // faster than real elapsed time.
+    var _last = -Infinity;
     return function() {
       var ms = Date.now() - (globalThis.performance.timeOrigin || 0);
-      if (ms > _last) _last = ms;
-      else _last += 0.001; // same/earlier ms: advance by a tiny epsilon
+      if (ms < _last) return _last;
+      _last = ms;
       return _last;
     };
   })(),

crates/obscura-js/src/runtime.rs

@@ -1330,6 +1330,20 @@ mod tests {
         assert_eq!(violations.as_f64(), Some(0.0), "performance.now() went backwards");
     }
 
+    #[test]
+    fn performance_now_does_not_outrun_elapsed_time() {
+        let mut rt = setup_runtime("<html><body></body></html>");
+        let lead = rt
+            .evaluate(
+                "(function(){for(var i=0;i<500000;i++)performance.now(); return performance.now()-(Date.now()-performance.timeOrigin);})()",
+            )
+            .unwrap();
+        assert!(
+            lead.as_f64().unwrap() <= 1.0,
+            "performance.now() advanced ahead of elapsed time: {lead}"
+        );
+    }
+
     #[test]
     fn test_document_title() {
         let mut rt = setup_runtime("<html><head><title>Test</title></head><body></body></html>");