Sometimes the dropdown width is too narrow. Possibly related to RT-28876.
Here's a test case, follow the instructions in the UI.

/* ComboBox dropdown width issue */
package comboboxwidth;

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Label;
import javafx.scene.layout.HBox;
import javafx.scene.layout.Priority;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class ComboBoxWidth extends Application {
    
    @Override
    public void start(Stage primaryStage) {
        ComboBox<String> combo = new ComboBox<>();
        combo.setMaxWidth(Double.MAX_VALUE);
        combo.getItems().addAll("1","2","3","4","5","6","7","8","9","10");
        combo.setEditable(true);
        VBox vbox = new VBox(4);
        vbox.setPadding(new Insets(4));
        HBox hbox = new HBox(4);
        HBox.setHgrow(combo, Priority.ALWAYS);
        hbox.getChildren().addAll(new Label("ComboBox:"), combo);
        vbox.getChildren().addAll(
                new Label("Open the drop down.\n"
                        + "Stretch this window wider.\n"
                        + "Open the drop down again.\n"
                        + "Then open the drop down a third time.\n"
                        + "Immediately after the the resize, the\n"
                        + "comboBox's drop down is the wrong size."),
                hbox,
                new Label(System.getProperty("java.version"))
        );
        Scene scene = new Scene(vbox, 300, 250);
        primaryStage.setTitle("ComboBox Width Test");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static void main(String[] args) { launch(args); }
}