Bug 1656786 - Make <select> list have 'overflow:auto' by default to indicate its scrollable. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D85874

dom/events/test/test_wheel_zoom_on_form_controls.html

@@ -15,6 +15,14 @@
 <input id="input" style="border: 10px solid green;"><br>
 <textarea id="textarea" style="border: 10px solid green;"></textarea><br>
 <select id="select"><option></option></select>
+<select id="list" size=4>
+  <option>XXXXXXXXXX</option>
+  <option>XXXXXXXXXX</option>
+  <option>XXXXXXXXXX</option>
+  <option>XXXXXXXXXX</option>
+  <option>XXXXXXXXXX</option>
+  <option>XXXXXXXXXX</option>
+</select>
 <script>
 
   async function testControl(id) {
@@ -43,6 +51,7 @@
     await testControl("input");
     await testControl("textarea");
     await testControl("select");
+    await testControl("list");
     SimpleTest.finish();
   }
 

layout/style/res/forms.css

@@ -246,6 +246,7 @@ select {
 }
 
 select:-moz-select-list-box {
+  overflow: auto;
   background-color: -moz-Field;
   color: -moz-FieldText;
   vertical-align: text-bottom;