Address review: no-JS fallback and NaN guard

- Scope the hiding rule to a class the script sets on load, so a failed
  or disabled script leaves the fields visible in their rendered position
  instead of hidden permanently
- Guard snapToStep against non-finite parses, which previously could
  write NaN:NaN back into the input

Author: dkastl

assets/javascripts/issue_datetime.js

@@ -8,14 +8,24 @@
 (function () {
   'use strict';
 
+  // Marks that this script actually loaded. The stylesheet only hides the
+  // detached container when this class is present, so a failed or disabled
+  // script leaves the fields visible where they were rendered instead of
+  // hiding them forever.
+  document.documentElement.classList.add('issue-datetime-js');
+
   function snapToStep(input) {
     var step = parseInt(input.getAttribute('step'), 10);
     if (!step || !input.value) return;
 
     var parts = input.value.split(':');
     if (parts.length < 2) return;
 
-    var minutes = parseInt(parts[0], 10) * 60 + parseInt(parts[1], 10);
+    var hours = parseInt(parts[0], 10);
+    var mins = parseInt(parts[1], 10);
+    if (!isFinite(hours) || !isFinite(mins)) return;
+
+    var minutes = hours * 60 + mins;
     var stepMinutes = step / 60;
     if (!stepMinutes) return;
 

assets/stylesheets/issue_datetime.css

@@ -1,6 +1,8 @@
-/* Hide the detached fields until issue_datetime.js has placed them, so they
-   do not flash at the bottom of the form on load. */
-#issue-datetime-fields.issue-datetime-pending {
+/* Hide the detached fields until issue_datetime.js has placed them, so they do
+   not flash at the bottom of the form on load. Scoped to the class that script
+   sets on load: if the script is missing or disabled, the fields stay visible
+   and usable in their rendered position rather than hidden for good. */
+.issue-datetime-js #issue-datetime-fields.issue-datetime-pending {
   display: none;
 }