[MINOR] Remove orphaned login modal backdrops before driving the navbar in Selenium tests

zeppelin-integration/src/test/java/org/apache/zeppelin/AbstractZeppelinIT.java

@@ -116,6 +116,7 @@ protected void authenticationUser(String userName, String password) {
    */
   private void dismissLoginModal() {
     if (loginModalClosed(MODAL_CLOSE_TIMEOUT_SEC)) {
+      removeOrphanedModalBackdrops();
       return;
     }
     LOGGER.warn("Login modal still displayed after {}s, taking it down from the page",
@@ -131,6 +132,44 @@ private void dismissLoginModal() {
     }
   }
 
+  // Bootstrap 3 removes a backdrop through this.$backdrop,
+  // so reopening the modal mid-transition orphans the earlier backdrop, which blocks every click.
+  // A hidden modal can keep its "in" class, so only a displayed modal owns a backdrop.
+  private void removeOrphanedModalBackdrops() {
+    Object removed = ((JavascriptExecutor) manager.getWebDriver()).executeScript(
+        "var backdrops = document.querySelectorAll('.modal-backdrop');"
+            + "var modals = document.querySelectorAll('.modal.in');"
+            + "for (var m = 0; m < modals.length; m++) {"
+            + "  if (window.getComputedStyle(modals[m]).display !== 'none') {"
+            + "    return null;"
+            + "  }"
+            + "}"
+            + "if (backdrops.length === 0) {"
+            + "  return null;"
+            + "}"
+            + "var classes = [];"
+            + "for (var i = 0; i < backdrops.length; i++) {"
+            + "  classes.push(backdrops[i].className);"
+            + "  backdrops[i].parentNode.removeChild(backdrops[i]);"
+            + "}"
+            + "return classes.join(', ');");
+    if (removed != null) {
+      LOGGER.warn("Removed modal backdrops left behind after the login modal closed: {}", removed);
+    }
+  }
+
+  private String describeModalState() {
+    Object state = ((JavascriptExecutor) manager.getWebDriver()).executeScript(
+        "var modal = document.getElementById('loginModal');"
+            + "var backdrops = document.querySelectorAll('.modal-backdrop');"
+            + "var classes = [];"
+            + "for (var i = 0; i < backdrops.length; i++) { classes.push(backdrops[i].className); }"
+            + "var modalState = modal"
+            + "    ? modal.className + ' display=' + modal.style.display : 'absent';"
+            + "return 'loginModal=' + modalState + ', backdrops=[' + classes.join(', ') + ']';");
+    return String.valueOf(state);
+  }
+
   /** Returns true once the login modal is hidden or gone, false if it is still displayed. */
   private boolean loginModalClosed(final long timeWait) {
     try {
@@ -213,7 +252,8 @@ protected void logoutUser(String userName) throws URISyntaxException {
       // login.controller.js re-opens it one second later, so it can appear between the
       // wait above and this click. An intercepted click never reached the menu, so the
       // dropdown is still closed and opening it again is safe.
-      LOGGER.warn("Navbar user menu click was intercepted, retrying once", e);
+      LOGGER.warn("Navbar user menu click was intercepted ({}), retrying once",
+          describeModalState(), e);
       dismissLoginModal();
       clickableWait(userMenu, MAX_BROWSER_TIMEOUT_SEC).click();
     }