Expressions: the kept layers are bounded like the runtime's scripts

crates/expr/src/prelude.js

@@ -31,6 +31,7 @@ function __begin(t, v, c, l, path, uid, idx, fd, ti, tt, sv) {
   __seed = 0;
   __timeless = false;
   __projectInfo = null;
+  __layersTrim();
 }
 
 function __finish(r) {
@@ -563,13 +564,25 @@ function Layer(c, l) {
 
 // Layer `l` of comp `c`, made once per runtime: a callable Layer is dearer to make than a plain
 // object, so it is kept, and its info (`__info`) is read again in each evaluation.
-// The kept layers are out of an expression's reach.
-var __layer = (function () {
-  var layers = {};
-  return function (c, l) {
+// The kept layers are out of an expression's reach, and bounded like the runtime's scripts: once
+// `max` are kept, the layers after them aren't, and the next evaluation starts over
+// (`__layersTrim`, so a layer stays the same object for a whole evaluation).
+var __layer, __layersTrim;
+(function () {
+  var layers = {}, kept = 0, max = 4096;
+  __layer = function (c, l) {
     // By comp, then layer: a `c + ':' + l` key costs more to build than the lookup saves.
-    var m = layers[c] || (layers[c] = {});
-    return m[l] || (m[l] = new Layer(c, l));
+    var m = layers[c], f = m && m[l];
+    if (f) return f;
+    f = new Layer(c, l);
+    if (kept < max) {
+      (m || (layers[c] = {}))[l] = f;
+      kept++;
+    }
+    return f;
+  };
+  __layersTrim = function () {
+    if (kept >= max) { layers = {}; kept = 0; }
   };
 })();