Fix MR target branch selector dropdown placement cut-off

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/31271

When the dropdown items are too wide, constrain the dropdown to the
offsetParent. Otherwise, let it naturally flow as Select2 wants.

app/assets/javascripts/dispatcher.js

@@ -49,6 +49,7 @@ import UserCallout from './user_callout';
 import { ProtectedTagCreate, ProtectedTagEditList } from './protected_tags';
 import ShortcutsWiki from './shortcuts_wiki';
 import BlobViewer from './blob/viewer/index';
+import AutoWidthDropdownSelect from './issuable/auto_width_dropdown_select';
 const ShortcutsBlob = require('./shortcuts_blob');
@@ -186,6 +187,7 @@ const ShortcutsBlob = require('./shortcuts_blob');
           new LabelsSelect();
           new MilestoneSelect();
           new gl.IssuableTemplateSelectors();
+          new AutoWidthDropdownSelect($('.js-target-branch-select')).init();
           break;
         case 'projects:tags:new':
           new ZenMode();

app/assets/javascripts/issuable/auto_width_dropdown_select.js

+let instanceCount = 0;
+
+class AutoWidthDropdownSelect {
+  constructor(selectElement) {
+    this.$selectElement = $(selectElement);
+    this.dropdownClass = `js-auto-width-select-dropdown-${instanceCount}`;
+    instanceCount += 1;
+  }
+
+  init() {
+    const dropdownClass = this.dropdownClass;
+    this.$selectElement.select2({
+      dropdownCssClass: dropdownClass,
+      dropdownCss() {
+        let resultantWidth = 'auto';
+        const $dropdown = $(`.${dropdownClass}`);
+
+        // We have to look at the parent because
+        // `offsetParent` on a `display: none;` is `null`
+        const offsetParentWidth = $(this).parent().offsetParent().width();
+        // Reset any width to let it naturally flow
+        $dropdown.css('width', 'auto');
+        if ($dropdown.outerWidth(false) > offsetParentWidth) {
+          resultantWidth = offsetParentWidth;
+        }
+
+        return {
+          width: resultantWidth,
+          maxWidth: offsetParentWidth,
+        };
+      },
+    });
+
+    return this;
+  }
+}
+
+export default AutoWidthDropdownSelect;

app/assets/stylesheets/pages/merge_requests.scss

@@ -482,6 +482,10 @@
   }
 }
+.target-branch-select-dropdown-container {
+  position: relative;
+}
+
 .assign-to-me-link {
   padding-left: 12px;
   white-space: nowrap;

app/views/shared/issuable/form/_branch_chooser.html.haml

@@ -10,12 +10,16 @@
     = form.label :source_branch, class: 'control-label'
     .col-sm-10
       .issuable-form-select-holder
-        = form.select(:source_branch, [issuable.source_branch], {}, { class: 'source_branch select2 span2', disabled: true })
+        = form.select(:source_branch, [issuable.source_branch], {}, { class: 'source_branch select2', disabled: true })
 .form-group
   = form.label :target_branch, class: 'control-label'
-  .col-sm-10
+  .col-sm-10.target-branch-select-dropdown-container
     .issuable-form-select-holder
-      = form.select(:target_branch, issuable.target_branches, { include_blank: true }, { class: 'target_branch select2 span2', disabled: issuable.new_record?, data: { placeholder: "Select branch" }})
+      = form.select(:target_branch, issuable.target_branches,
+        { include_blank: true },
+        { class: 'target_branch js-target-branch-select',
+          disabled: issuable.new_record?,
+          data: { placeholder: "Select branch" }})
     - if issuable.new_record?
        
       = link_to 'Change branches', mr_change_branches_path(issuable)