Merged column headers should work within declarative Grid. (#12206)

- null check to prevent NPE from the merged column headers
- convert to internal columnIds for merge handling
- use correct cell in merge handling
- switch away from streams in merge handling for easier readability
- regression test

Fixes: #10464

1 files changed, 19 insertions, 0 deletions

diff --git a/uitest/src/test/java/com/vaadin/tests/declarative/DeclarativeGridTest.java b/uitest/src/test/java/com/vaadin/tests/declarative/DeclarativeGridTest.java
new file mode 100644
index 0000000000..5babbae75b
--- /dev/null
+++ b/uitest/src/test/java/com/vaadin/tests/declarative/DeclarativeGridTest.java
@@ -0,0 +1,19 @@
+package com.vaadin.tests.declarative;
+
+import org.junit.Test;
+
+import com.vaadin.testbench.By;
+import com.vaadin.testbench.elements.GridElement;
+import com.vaadin.tests.tb3.MultiBrowserTest;
+
+public class DeclarativeGridTest extends MultiBrowserTest {
+
+    @Test
+    public void testMergedHeaderCell() {
+        openTestURL();
+        waitForElementPresent(By.className("v-label"));
+        // ensure the grid gets loaded and has the merged header
+        GridElement grid = $(GridElement.class).first();
+        grid.getHeaderCellByCaption("Project and Status");
+    }
+}