ADDITIONAL SYSTEM INFORMATION :
JavaFX version 15 and 16

A DESCRIPTION OF THE PROBLEM :
index computation and rendering of combobox has issues when removing multiple items at once with itemsProperty().get().removeAll(Collection). This is because of a difference in how the list change is computed and how the selected index is updated.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Precondition:
- combobox with three items
- middle item selected
Action:
- remove first and last item via combox.itemsProperty().get().removeAll(...) (in example: button "Trigger bug")

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
middle item stays selected and rendered, selected index is 0 (first and only item)
ACTUAL -
middle item stays selected, but rendered text is empty, since selected index is -1.

---------- BEGIN SOURCE ----------
package test;

import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.ComboBox;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class App extends Application {

  private ComboBox<String> comboBox;

  @Override
  public void start(Stage primaryStage) throws Exception {
    var root = new StackPane();
    var scene = new Scene(root);
    primaryStage.setScene(scene);
    var box = new VBox();
    var buttonBug = new Button();
    var buttonWorkaround = new Button();
    var buttonReset = new Button();
    comboBox = new ComboBox<>();
    comboBox.itemsProperty().set(FXCollections.observableArrayList(List.of("foo", "bar", "baz")));
    comboBox.valueProperty().set("bar");
    buttonBug.textProperty().set("Trigger bug");
    buttonBug.onActionProperty().set(e -> comboBox.itemsProperty().get().removeAll(getAllButSelected()));
    buttonWorkaround.textProperty().set("Workaround bug");
    buttonWorkaround.onActionProperty().set(e -> getAllButSelected().forEach(comboBox.itemsProperty().get()::remove));
    buttonReset.textProperty().set("Reset");
    buttonReset.onActionProperty().set(e -> {
      comboBox.itemsProperty().get().setAll("foo", "bar", "baz");
      comboBox.valueProperty().set("bar");
    });
    box.getChildren().addAll(comboBox, buttonBug, buttonWorkaround, buttonReset);
    root.getChildren().add(box);
    primaryStage.show();
  }

  private Set<String> getAllButSelected() {
    var result = new HashSet<>(comboBox.itemsProperty().get());
    var selected = comboBox.valueProperty().get();
    result.remove(selected);
    return result;
  }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
remove items one by one instead of with removeAll (button "Workaround bug")

FREQUENCY : always