Update ComboBox popup position comparison to use correct top value. (#12041)

Fixes #12029

client/src/main/java/com/vaadin/client/ui/VComboBox.java

                 // ComboBox itself may be incorrectly positioned, don't try to
                 // adjust horizontal popup position yet. Earlier width
                 // calculations must be performed anyway to avoid flickering.
-                if (top != topPosition) {
+                if (top != getAbsoluteTop()) {
                     // Variable 'left' still contains the original popupLeft,
                     // 'top' has been updated, thus vertical position needs
                     // adjusting.
             }
 
             // Only update the position if it has changed.
-            if (top != topPosition || left != getPopupLeft()) {
+            if (top != getAbsoluteTop() || left != getPopupLeft()) {
                 setPopupPosition(left, top);
             }
             menu.scrollSelectionIntoView();

uitest/src/main/java/com/vaadin/tests/components/combobox/ComboBoxAtBottomEdgeWithinHorizontalLayout.java

     @Override
     protected void setup(VaadinRequest request) {
         ComboBox<Integer> comboBox = new ComboBox<>();
-        comboBox.setItems(Arrays.asList(100, 200, 300, 400, 500));
+        comboBox.setItems(Arrays.asList(102, 205, 302, 402, 500));
 
         HorizontalLayout horizontalLayout = new HorizontalLayout();
         horizontalLayout.addComponent(comboBox);

uitest/src/test/java/com/vaadin/tests/components/combobox/ComboBoxAtBottomEdgeWithinHorizontalLayoutTest.java

 import org.junit.Test;
 import org.openqa.selenium.WebElement;
 
+import com.vaadin.testbench.By;
 import com.vaadin.testbench.elements.ComboBoxElement;
 import com.vaadin.tests.tb3.MultiBrowserTest;
 
                 cbBottom, popupBottom), cbBottom, popupBottom);
     }
 
+    @Test
+    public void ensurePopupPositionUpdatesWhenFiltered() {
+        openTestURL();
+
+        ComboBoxElement cb = $(ComboBoxElement.class).first();
+        cb.openPopup();
+        WebElement popup = cb.getSuggestionPopup();
+
+        int initialTop = popup.getLocation().getY();
+
+        // filter a bit
+        cb.findElement(By.vaadin("#textbox")).sendKeys("2");
+        int updatedTop = popup.getLocation().getY();
+        assertLessThan(String.format(
+                "Popup should be repositioned when "
+                        + "filtered. Initial: %s, Updated: %s",
+                initialTop, updatedTop), initialTop, updatedTop);
+        int cbBottom = cb.getLocation().getY() + cb.getSize().getHeight();
+        assertGreaterOrEqual(String.format(
+                "Popup should still open above the ComboBox when "
+                        + "filtered a bit. ComboBox: %s, Popup: %s",
+                cbBottom, updatedTop), cbBottom, updatedTop);
+
+        // filter more
+        cb.clear();
+        cb.findElement(By.vaadin("#textbox")).sendKeys("1");
+        popup = cb.getSuggestionPopup();
+        updatedTop = popup.getLocation().getY();
+        assertLessThanOrEqual(String.format(
+                "Popup should open below the ComboBox when "
+                        + "filtered down to one result. ComboBox: %s, Popup: %s",
+                cbBottom, updatedTop), cbBottom, updatedTop);
+    }
+
 }