When a bold text is set for month/year spinner the width spinner width varies when the month is cahnged by pressing the related arrow button. Sometimes the complete popup width varies - see test case below.

public class CSSDatePickerTest extends Application
{
  public static void main(String[] args)
  {
    Application.launch(args);
  }

  @Override
  public void start(Stage stage)
  {
    System.err.println(System.getProperty("javafx.runtime.version"));
    Application.setUserAgentStylesheet(getClass().getResource("/test/datepicker/style.css").toExternalForm());
    
    Parent content = createContent();
    Scene scene = new Scene(content, 500, 500);
    stage.setScene(scene);
    stage.show();
  }
    
  private Parent createContent()
  {
    VBox vbox = new VBox(10);
    vbox.getChildren().add(createDatePicker(false, false));
    vbox.getChildren().add(createDatePicker(true, false));
    vbox.getChildren().add(createDatePicker(false, true));
    return vbox;
  }

  private DatePicker createDatePicker(boolean showWeekNumbers, boolean disabled)
  {
    DatePicker dp = new DatePicker();
    dp.setDisable(disabled);
    dp.setShowWeekNumbers(showWeekNumbers);
    return dp;
  }
}

style.css:

 .root {
-fx-font: 11 Tahoma;
    -fx-text-fill: black;
}

 .text {
    -fx-text-fill: #F00;
    -fx-font-smoothing-type: lcd;
}

.combo-box-base > .arrow-button {
   -fx-background-color: red;
   -fx-padding: 0 4;
  }

.combo-box-base > .arrow-button > .arrow {
    -fx-background-color: black;
    -fx-background-insets: 0 0 0 0;
    -fx-padding: 2 4;
    -fx-shape: "M 0 0 H 7 L 3.5 4 z";
}


/*******************************************************************************
 * *
 * DatePicker *
 * *
 ******************************************************************************/

.date-picker-popup {
    -fx-padding: 1;
    -fx-background-color: lightgray;
}

.date-picker-popup > .month-year-pane {
    -fx-padding: 4;
    -fx-background-color: #EFEFEF;
}

.date-picker-popup > .month-year-pane > .spinner {
    -fx-spacing: 4;
    -fx-alignment: CENTER;
    -fx-fill-height: false;
}

.date-picker-popup > .month-year-pane > .spinner > .spinner-label {
    -fx-font-size: 1.083333em;
    -fx-alignment: CENTER;
    -fx-text-fill: #606060;
    -fx-font-weight: bold;
}

.date-picker-popup > .month-year-pane > .spinner > .left-button {
    -fx-padding: 4 6;
    -fx-background-color: gray;
}

.date-picker-popup > .month-year-pane > .spinner > .right-button {
    -fx-padding: 4 6;
    -fx-background-color: gray;
}

.date-picker-popup > .month-year-pane > .spinner > .button > .left-arrow {
    -fx-background-color: red;
    -fx-background-insets: 0;
    -fx-padding: 0.333em 0.167em 0.333em 0.167em;
    -fx-shape: "M5.997,5.072L5.995,6.501l-2.998-4l2.998-4l0.002,1.43l-1.976,2.57L5.997,5.072z";
    -fx-scale-shape: true;
}

.date-picker-popup > .month-year-pane > .spinner > .button > .right-arrow {
    -fx-background-color: red;
    -fx-background-insets: 0;
    -fx-padding: 0.333em 0.167em 0.333em 0.167em;
    -fx-shape: "M2.998-0.07L3-1.499l2.998,4L3,6.501l-0.002-1.43l1.976-2.57L2.998-0.07z";
    -fx-scale-shape: true;
}

.date-picker-popup > * > .cell {
    -fx-padding: 0 7;
    -fx-alignment: BASELINE_CENTER;
}

.date-picker-popup > * > .cell:disabled > * {
    /* This is set on the cell's children so the border is not affected. */
    -fx-opacity: 0.4;
}

.date-picker-popup > * > .week-number-header-cell,
.date-picker-popup > * > .day-name-cell {
    -fx-font-size: 0.833333em;
    -fx-padding: 0;
    -fx-text-fill: #0070C0;
    -fx-font-weight: bold;
}

.date-picker-popup > * > .week-number-cell {
    -fx-font-size: 0.75em;
    -fx-padding: 0;
}

.date-picker-popup > * > .day-cell {
    -fx-text-fill: black;
    -fx-border-color: lightgray;
    -fx-border-width: 1;
    -fx-border-insets: 0;
}

.date-picker-popup > * > .hijrah-day-cell {
    -fx-alignment: TOP_LEFT;
    -fx-cell-size: 2.75em;
}

.date-picker-popup > * > .day-cell:focused {
    -fx-background-color: #CCE8FF;
}

.date-picker-popup > * > .day-cell:hover {
    -fx-background-color: #EFEFEF;
}

.date-picker-popup > * > .day-cell > .secondary-text {
    -fx-font-size: 1.0em;
    -fx-fill: #ff5050;
}

.date-picker-popup > * > .today {
    -fx-background-insets: 1;
    -fx-border-color: #606060;
    -fx-border-insets: 1;
}

.date-picker-popup > * > .today:focused {
    -fx-background-color: #CCE8FF;
}

.date-picker-popup > * > .selected {
    -fx-background-color: #CCE8FF;
}

.date-picker-popup > * > .previous-month,
.date-picker-popup > * > .next-month {
    -fx-text-fill: #aaaaaa;
}

.date-picker-popup > * > .previous-month > .secondary-text,
.date-picker-popup > * > .next-month > .secondary-text {
    -fx-fill: #aaaaaa;
}