fix: Correct negative zero logic for +/- and update test

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

calculator/calculator.app.js

@@ -532,10 +532,23 @@ function buttonPress(val) {
       break;
     case 'N':
       if (results != null && !hasPressedNumber) {
-        displayOutput(results *= -1);
+        if (results === 0) {
+          results = (1/results === -Infinity || Object.is(results, -0)) ? 0 : -0;
+        } else {
+          results *= -1;
+        }
+        displayOutput(results);
         prevExpression = "-(" + prevExpression + ")";
       } else {
-        displayOutput(currNumber *= -1);
+        if (currNumber === null) currNumber = '0';
+        var num = parseFloat(currNumber);
+        if (num === 0) {
+          // Toggle between 0 and -0
+          currNumber = (1/currNumber === -Infinity || Object.is(currNumber, -0)) ? '0' : -0;
+        } else {
+          currNumber = num * -1;
+        }
+        displayOutput(currNumber);
         currExpression = "-(" + currExpression + ")";
       }
       break;

calculator/test.js

@@ -493,11 +493,11 @@ test('Negative zero handling', () => {
   press('0N'); // get -0
   checkDisplay(-0);
   press('*5');
-  checkDisplay(-0);
+  checkDisplay('5'); // after typing 5, display should be 5
   press('=');
-  checkDisplay(-0);
+  checkDisplay(-0); // -0 * 5 = -0
   buttonPress('R'); buttonPress('R');
-  press('5*-0=');
+  press('5*0N='); // 5 * -0 = -0
   checkDisplay(-0);
 });