Fix Grid header merging on init (#9688)

Fixes #9686

client/src/main/java/com/vaadin/client/connectors/grid/ColumnConnector.java

@@ -20,6 +20,7 @@ import com.vaadin.client.annotations.OnStateChange;
 import com.vaadin.client.connectors.AbstractRendererConnector;
 import com.vaadin.client.extensions.AbstractExtensionConnector;
 import com.vaadin.client.widgets.Grid.Column;
+import com.vaadin.client.widgets.Grid.HeaderCell;
 import com.vaadin.shared.data.DataCommunicatorConstants;
 import com.vaadin.shared.ui.Connect;
 import com.vaadin.shared.ui.grid.ColumnState;
@@ -48,6 +49,11 @@ public class ColumnConnector extends AbstractExtensionConnector {
         public String getConnectorId() {
             return connectorId;
         }
+
+        @Override
+        protected void setDefaultHeaderContent(HeaderCell cell) {
+            // NO-OP, Server takes care of header contents.
+        }
     }
 
     private CustomColumn column;

client/src/main/java/com/vaadin/client/widgets/Grid.java

@@ -4853,7 +4853,7 @@ public class Grid<T> extends ResizeComposite implements HasSelectionHandlers<T>,
         private void updateHeader() {
             HeaderRow row = grid.getHeader().getDefaultRow();
             if (row != null) {
-                row.getCell(this).setText(headerCaption);
+                setDefaultHeaderContent(row.getCell(this));
                 if (isHidable()) {
                     grid.columnHider.updateHidingToggle(this);
                 }

uitest/src/main/java/com/vaadin/tests/components/grid/GridComponents.java

@@ -25,7 +25,8 @@ public class GridComponents extends AbstractTestUIWithLog {
     @Override
     protected void setup(VaadinRequest request) {
         Grid<String> grid = new Grid<>();
-        grid.addColumn(string -> new Label(string), new ComponentRenderer());
+        grid.addColumn(string -> new Label(string), new ComponentRenderer())
+                .setCaption("Label");
         grid.addComponentColumn(string -> {
             if (textFields.containsKey(string)) {
                 log("Reusing old text field for: " + string);
@@ -41,7 +42,7 @@ public class GridComponents extends AbstractTestUIWithLog {
                 textFields.put(string, textField);
             });
             return textField;
-        });
+        }).setId("textField").setCaption("TextField");
         grid.addColumn(string -> {
             Button button = new Button("Click Me!",
                     e -> Notification.show(
@@ -49,10 +50,13 @@ public class GridComponents extends AbstractTestUIWithLog {
                             Type.WARNING_MESSAGE));
             button.setId(string.replace(' ', '_').toLowerCase());
             return button;
-        }, new ComponentRenderer());
+        }, new ComponentRenderer()).setId("button").setCaption("Button");
         // make sure the buttons and focus outlines fit completely in a row
         grid.setRowHeight(40);
 
+        grid.getDefaultHeaderRow().join("textField", "button")
+                .setText("Other Components");
+
         addComponent(grid);
         grid.setSizeFull();
 

uitest/src/test/java/com/vaadin/tests/components/grid/GridComponentsTest.java

@@ -82,6 +82,15 @@ public class GridComponentsTest extends MultiBrowserTest {
         assertRowExists(999, "Row 999");
     }
 
+    @Test
+    public void testHeaders() {
+        openTestURL();
+        GridElement grid = $(GridElement.class).first();
+        Assert.assertEquals("Label", grid.getHeaderCell(0, 0).getText());
+        Assert.assertEquals("Other Components",
+                grid.getHeaderCell(0, 1).getText());
+    }
+
     private void assertRowExists(int i, String string) {
         GridRowElement row = $(GridElement.class).first().getRow(i);
         Assert.assertEquals("Label text did not match", string,