[ZEPPELIN-6409][ZEPPELIN-6400] Fix Selenium integration test flakiness after

## Summary

- **`authenticationUser()`**: Replace `sleep(1000)` with explicit wait for the navbar user dropdown to appear (proves login succeeded and AngularJS digest is done), then force-dismiss any lingering Bootstrap modal backdrop via jQuery
- **`logoutUser()`**: Use `clickableWait()` instead of raw `findElement()` for robust element interaction; wrap modal close button in try-catch
- **`testAngularRunParagraph()`**: Fix race condition where `waitForParagraph("FINISHED")` matched the *previous* run's state. Use `stalenessOf` to detect paragraph output refresh, then `visibilityWait` for the new element, and JavaScript click to bypass ng-click overlay issues

## Context

After ZEPPELIN-6400 moved `ZeppelinConfiguration` from `zeppelin-interpreter` to `zeppelin-zengine`, `RemoteInterpreterServer.init()` changed from loading config via `ZeppelinConfiguration.load()` + overlay to pure `Properties`-based initialization. This subtle timing change in interpreter startup exposed pre-existing Selenium test flakiness in the `test-selenium-with-spark-module-for-spark-3-5` CI job.

The CI failures were:
- `InterpreterModeActionsIT.testPerUserIsolatedAction` — `ElementClickInterceptedException` because login modal was still visible when clicking the navigation dropdown
- `ZeppelinIT.testAngularRunParagraph` — `TimeoutException` due to race condition: after re-running an Angular paragraph, the old output element was matched before the new one rendered

## Test plan

- [x] CI: `frontend.yml` — `test-selenium-with-spark-module-for-spark-3-5` job passes
- [x] CI: `frontend.yml` — All other E2E jobs remain green (Playwright auth failure is unrelated Firefox keyboard shortcut flakiness)

Closes #5209 from jongyoul/ZEPPELIN-6409-fix-selenium-tests.

Signed-off-by: Jongyoul Lee <jongyoul@gmail.com>

Author: jongyoul

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

@@ -61,21 +61,38 @@ protected void authenticationUser(String userName, String password) {
         By.xpath("//*[@id='loginModalContent']//button[contains(.,'Login')]"),
         MAX_BROWSER_TIMEOUT_SEC).click();
 
-    ZeppelinITUtils.sleep(1000, false);
+    // Wait for the logged-in navbar user dropdown to appear (indicates login completed
+    // and Angular digest cycle has updated the DOM), then dismiss any leftover modal overlay
+    visibilityWait(
+        By.xpath("//div[contains(@class, 'navbar-collapse')]//li//button[contains(@class, 'nav-btn dropdown-toggle ng-scope')]"),
+        MAX_BROWSER_TIMEOUT_SEC);
+    try {
+      ((JavascriptExecutor) manager.getWebDriver()).executeScript(
+          "$('.modal-backdrop').remove(); $('#loginModal').modal('hide');");
+    } catch (Exception e) {
+      // ignore if jQuery/Bootstrap not ready
+    }
+    ZeppelinITUtils.sleep(500, false);
   }
 
   protected void logoutUser(String userName) throws URISyntaxException {
     ZeppelinITUtils.sleep(500, false);
-    manager.getWebDriver().findElement(
-        By.xpath("//div[contains(@class, 'navbar-collapse')]//li[contains(.,'" + userName + "')]")).click();
+    clickableWait(
+        By.xpath("//div[contains(@class, 'navbar-collapse')]//li[contains(.,'" + userName + "')]"),
+        MAX_BROWSER_TIMEOUT_SEC).click();
     ZeppelinITUtils.sleep(500, false);
-    manager.getWebDriver().findElement(
-        By.xpath("//div[contains(@class, 'navbar-collapse')]//li[contains(.,'" + userName + "')]//a[@ng-click='navbar.logout()']")).click();
+    clickableWait(
+        By.xpath("//div[contains(@class, 'navbar-collapse')]//li[contains(.,'" + userName + "')]//a[@ng-click='navbar.logout()']"),
+        MAX_BROWSER_TIMEOUT_SEC).click();
     ZeppelinITUtils.sleep(2000, false);
-    if (manager.getWebDriver().findElement(
-        By.xpath("//*[@id='loginModal']//div[contains(@class, 'modal-header')]/button")).isDisplayed()) {
-      manager.getWebDriver().findElement(
-          By.xpath("//*[@id='loginModal']//div[contains(@class, 'modal-header')]/button")).click();
+    try {
+      WebElement closeButton = manager.getWebDriver().findElement(
+          By.xpath("//*[@id='loginModal']//div[contains(@class, 'modal-header')]/button"));
+      if (closeButton.isDisplayed()) {
+        closeButton.click();
+      }
+    } catch (NoSuchElementException e) {
+      // login modal close button not found, which is fine
     }
     manager.getWebDriver().get(new URI(manager.getWebDriver().getCurrentUrl()).resolve("/classic/#/").toString());
     ZeppelinITUtils.sleep(500, false);

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

@@ -33,11 +33,15 @@
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
+import java.time.Duration;
 import org.openqa.selenium.By;
+import org.openqa.selenium.JavascriptExecutor;
 import org.openqa.selenium.Keys;
 import org.openqa.selenium.StaleElementReferenceException;
 import org.openqa.selenium.TimeoutException;
 import org.openqa.selenium.WebElement;
+import org.openqa.selenium.support.ui.ExpectedConditions;
+import org.openqa.selenium.support.ui.WebDriverWait;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -326,16 +330,31 @@ void testAngularRunParagraph() throws Exception {
               "%angular <div id=\\'angularRunParagraph\\' ng-click=\\'z.runParagraph(\""
                       + secondParagraphId.trim()
                       + "\")\\'>Run second paragraph</div>");
+
+      // Capture old output element before re-run to detect when it gets replaced
+      WebElement oldAngularDiv = manager.getWebDriver().findElement(By.xpath(
+              getParagraphXPath(1) + "//div[@id=\"angularRunParagraph\"]"));
+
       runParagraph(1);
+
+      // Wait for the old output element to become stale (proves the paragraph output
+      // was actually refreshed, avoiding race where waitForParagraph sees the old FINISHED state)
+      new WebDriverWait(manager.getWebDriver(), Duration.ofSeconds(MAX_BROWSER_TIMEOUT_SEC))
+              .until(ExpectedConditions.stalenessOf(oldAngularDiv));
+
       waitForParagraph(1, "FINISHED");
 
+      // Wait for new Angular output to render
+      WebElement newAngularDiv = visibilityWait(By.xpath(
+              getParagraphXPath(1) + "//div[@id=\"angularRunParagraph\"]"), MAX_BROWSER_TIMEOUT_SEC);
+
       // Set new text value for 2nd paragraph
       setTextOfParagraph(2, "%sh echo NEW_VALUE");
 
-      // Click on 1 paragraph to trigger z.runParagraph() function
-
+      // Click on Angular-rendered div to trigger z.runParagraph() function.
+      // Use JavaScript click to bypass overlay/clickability issues with ng-click elements.
+      ((JavascriptExecutor) manager.getWebDriver()).executeScript("arguments[0].click();", newAngularDiv);
       ZeppelinITUtils.sleep(1000, false);
-      clickAndWait(By.xpath(getParagraphXPath(1) + "//div[@id=\"angularRunParagraph\"]"));
 
       waitForParagraph(2, "FINISHED");