made "i" upvar inside FOR loop's C-Shape slot mutable by user script

HISTORY.md

@@ -6,6 +6,7 @@
     * accept a list of pixels in the SWITCH TO COSTUME block
 * **Notable Changes:**
 * **Notable Fixes:**
+    * made "i" upvar inside FOR loop's C-Shape slot mutable by user script
     * typing strings into the search-field again shows relevant blocks (regression from IME)
     * fixed project dialog's search-field behevior (regression from IME)
     * morphic collision detection off-by-1 fix, thanks, Dariusz!
@@ -23,6 +24,7 @@
 * objects: fixed #2485 (find blocks and text-entry mode feature)
 * gui: fixed ProjectDialog's search field behavior for IME 
 * threads: fixed MY PARTS so mutating the result list has no effect
+* threads: made "i" upvar inside FOR loop's C-Shape slot mutable by user script
 
 ### 2019-10-11
 * objects, threads: accept a list of pixels in the SWITCH TO COSTUME block

src/threads.js

@@ -2207,28 +2207,28 @@ Process.prototype.doForEach = function (upvar, list, script) {
     this.evaluate(script, new List([next]), true);
 };
 
-Process.prototype.doFor = function (upvar, start, end, script) {
+Process.prototype.doFor = function (upvar, start, end, script) { // +++
     // perform a script for every integer step between start and stop,
     // assigning the current iteration index to a variable with the
     // name specified in the "upvar" parameter, so it can be referenced
     // within the script.
 
-    var dta;
-    if (this.context.accumulator === null) {
-        this.context.accumulator = {
-            idx : Math.floor(start),
+    var vars = this.context.outerContext.variables,
+        dta = this.context.accumulator;
+    if (dta === null) {
+        dta = this.context.accumulator = {
             test : start < end ?
-                function () {return this.idx > end; }
-                    : function () {return this.idx < end; },
+                function () {return vars.getVar(upvar) > end; }
+                    : function () {return vars.getVar(upvar) < end; },
             step : start < end ? 1 : -1,
             parms : new List() // empty parameters, reusable to avoid GC
         };
+        vars.addVar(upvar);
+        vars.setVar(upvar, Math.floor(start));
+    } else {
+        vars.changeVar(upvar, dta.step);
     }
-    dta = this.context.accumulator;
-    this.context.outerContext.variables.addVar(upvar);
-    this.context.outerContext.variables.setVar(upvar, dta.idx);
     if (dta.test()) {return; }
-    dta.idx += dta.step;
     this.pushContext('doYield');
     this.pushContext();
     this.evaluate(script, dta.parms, true);