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diff --git a/js/src/jit-test/tests/debug/Environment-identity-03.js b/js/src/jit-test/tests/debug/Environment-identity-03.js
new file mode 100644
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+++ b/js/src/jit-test/tests/debug/Environment-identity-03.js
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+// Two Environments nested in the same runtime scope share the correct tail of their parent chains.
+
+// The compiler must be allowed to elide empty scopes and so forth, so this
+// test does not check the number of unshared Environments. Instead, each test
+// case identifies the expected innermost shared scope by the name of a
+// variable in it.
+
+var g = newGlobal({newCompartment: true});
+g.eval("function h() { debugger; }");
+var dbg = Debugger(g);
+var hits, name, shared, unshared;
+dbg.onDebuggerStatement = function (hframe) {
+    var frame = hframe.older;
+
+    // Find name in frame.environment.
+    var env, child = null;
+    for (env = frame.environment; env !== null; env = env.parent) {
+        if (env.names().indexOf(name) != -1)
+            break;
+        child = env;
+    }
+    assertEq(env !== null, true, "expected '" + name + "' to be in scope");
+    assertEq(env, frame.environment.find(name),
+             "env.find should find the same frame as the written out search");
+
+    if (hits === 0) {
+        // First hit.
+        shared = env;
+        unshared = child;
+    } else {
+        // Subsequent hit.
+        assertEq(env, shared, "the environment containing '" + name + "' should be shared");
+        assertEq(child === null || unshared === null || unshared !== child, true,
+                "environments nested within the one containing '" + name + "' should not be shared");
+    }
+    hits++;
+};
+
+function test(sharedName, expectedHits, code) {
+    hits = 0;
+    name = sharedName;
+    shared = unshared = undefined;
+    g.eval(code);
+    assertEq(hits, expectedHits);
+}
+
+// Basic test cases.
+//
+// (The stray "a = b" assignments in these tests are to inhibit the flat closure
+// optimization, which Environments expose. There's nothing really wrong with
+// the optimization or with the debugger exposing it, but that's not what we
+// want to test here.)
+
+test("q", 2, "let q = function (a) { h(); }; q(1); q(2);");
+test("a", 2, "q = function (a) { (function (b) { h(); a = b; })(2); h(); }; q(1);");
+test("a", 2, "q = function (a) { h(); return function (b) { h(); a = b; }; }; q(1)(2);");
+test("n", 3, "q = function (n) { for (var i = 0; i < n; i++) { { let j = i; h(); } } }; q(3);");
+
+// A function with long dynamic and static chains.
+var N = 80;
+
+var code = "function f" + N + "(a" + N + ") {\neval('a0 + a1'); h();\n}\n";
+for (var i = N; --i >= 0;) {
+    var call = "f" + (i + 1) + "(a" + i + " - 1);\n";
+    code = ("function f" + i + "(a" + i + ") {\n" +
+            code +
+            call +
+            "if (a" + i + " === 0) " + call +
+            "}\n");
+}
+
+g.eval(code);
+test("a0", 2, "f0(0);");
+test("a17", 2, "f0(17);");
+test("a" + (N-2), 2, "f0(" + (N-2) + ");");
+test("a" + (N-1), 2, "f0(" + (N-1) + ");");
+
+// A function with a short dynamic chain and a long static chain.
+N = 60;
+
+function DeepStaticShallowDynamic(i, n) {
+    var code = "function f" + i + "(a" + i + ") {\n";
+    if (i >= n)
+        code += "eval('a1 + a2'); h();\n";
+    else
+        code += "return " + DeepStaticShallowDynamic(i+1, n) + ";\n";
+    code += "}";
+    return code;
+}
+g.eval(DeepStaticShallowDynamic(1, N));
+
+function* range(start, stop) {
+    for (var i = start; i < stop; i++)
+        yield i;
+}
+
+function DSSDsplit(s) {
+    return ("var mid = f1" + [...range(0, s)].map((i) => "(" + i + ")").join("") + ";\n" +
+            "mid" +          [...range(s, N)].map((i) => "(" + i + ")").join("") + ";\n" +
+            "mid" +          [...range(s, N)].map((i) => "(" + i + ")").join("") + ";\n");
+}
+
+test("a1", 2, DSSDsplit(1));
+test("a17", 2, DSSDsplit(17));
+test("a" + (N-2), 2, DSSDsplit(N-2));
+test("a" + (N-1), 2, DSSDsplit(N-1));