Disable the dropdown instead of not showing it

To make the screen more stable. This prevents the tabs from being
pushed down when the dropdown is rendered and making the popover
anchor wrong.

app/assets/javascripts/pipeline_editor/components/file_nav/branch_switcher.vue

@@ -112,7 +112,7 @@ export default {
     isBranchesLoading() {
       return this.$apollo.queries.availableBranches.loading || this.isSearchingBranches;
     },
-    showBranchSwitcher() {
+    enableBranchSwitcher() {
       return this.branches.length > 0 || this.searchTerm.length > 0;
     },
   },
@@ -230,11 +230,11 @@ export default {
 
 <template>
   <gl-dropdown
-    v-if="showBranchSwitcher"
     v-gl-tooltip.hover
     :title="$options.i18n.dropdownHeader"
     :header-text="$options.i18n.dropdownHeader"
     :text="currentBranch"
+    :disabled="!enableBranchSwitcher"
     icon="branch"
     data-qa-selector="branch_selector_button"
     data-testid="branch-selector"

spec/frontend/pipeline_editor/components/file-nav/branch_switcher_spec.js

@@ -141,8 +141,8 @@ describe('Pipeline editor branch switcher', () => {
       createComponentWithApollo();
     });
 
-    it('does not render dropdown', () => {
-      expect(findDropdown().exists()).toBe(false);
+    it('disables the dropdown', () => {
+      expect(findDropdown().props('disabled')).toBe(true);
     });
   });
 
@@ -189,7 +189,7 @@ describe('Pipeline editor branch switcher', () => {
     });
 
     it('does not render dropdown', () => {
-      expect(findDropdown().exists()).toBe(false);
+      expect(findDropdown().props('disabled')).toBe(true);
     });
 
     it('shows an error message', () => {