focus the first flag reason upon opening the flag modal

Author: Oaphi

app/assets/javascripts/flags.js

@@ -55,10 +55,18 @@ $(() => {
     }
   });
 
+  QPixel.DOM?.watchClass('.js-flag-box.is-active', (target) => {
+    const reasonInput = target.querySelector('input[name="flag-reason"]');
+
+    if (QPixel.DOM?.isHTMLElement(reasonInput)) {
+      reasonInput.focus();
+    }
+  });
+
   QPixel.DOM?.watchClass('[id^="flag-comment"].is-active', (target) => {
     const reasonInput = target.querySelector('.js-comment-flag-reason');
 
-    if (reasonInput instanceof HTMLInputElement) {
+    if (QPixel.DOM?.isHTMLElement(reasonInput)) {
       reasonInput.focus();
     }
   });