FULL PRODUCT VERSION :
java version "1.8.0_102"
Java(TM) SE Runtime Environment (build 1.8.0_102-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.102-b14, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 10.0.10586]

A DESCRIPTION OF THE PROBLEM :
error with fxml and css.
unexpected change of button font weight.
it occurs while clicking around buttons.
fxml and css are listed below

/**myform.css*/
.root {
    /*-fx-background-color: beige;*/
    /*-fx-font-family: "Arial";*/
    /*-fx-font-weight: normal;*/
}

.button {
    -fx-text-fill: white;
    -fx-font-weight: bold;
    -fx-background-color: linear-gradient(#61a2b1, #2A5058);
    -fx-effect: dropshadow(three-pass-box, rgba(0, 0, 0, 0.6), 5, 0.0, 0, 1);
}

.button:hover {
    -fx-background-color: linear-gradient(#43808c, #2A5058);
}

.button:pressed {
    -fx-background-color: #2A5058;
}

#login-title-label {
    -fx-font-size: 32px;
    -fx-font-family: "Times New Roman";
    -fx-fill: #818181;
    -fx-effect: innershadow(three-pass-box, rgba(0, 0, 0, 0.7), 6, 0.0, 0, 2);
}

.label {
}



/**myform.fxml*/
<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.chart.*?>
<?import javafx.collections.*?>
<?import javafx.scene.text.*?>
<?import java.net.*?>
<?import javafx.geometry.*?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import java.net.URL?>
<?import javafx.geometry.Insets?>
<?import javafx.collections.FXCollections?>

<TabPane id="main-pane" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="561.0" prefWidth="808.0" tabClosingPolicy="UNAVAILABLE" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">
    <tabs>
     
        <Tab text="Задачи">
            <content>
                <HBox prefHeight="100.0" prefWidth="200.0">
                    <children>
                        <TableView prefHeight="532.0" prefWidth="435.0">
                            <columns>
                                <TableColumn prefWidth="75.0" text="Название" />
                                <TableColumn prefWidth="96.0" text="Состояние" />
                                <TableColumn prefWidth="90.0" text="Осталось" />
                                <TableColumn prefWidth="96.0" text="Прогресс" />
                            </columns>
                        </TableView>
                        <GridPane prefHeight="532.0" prefWidth="365.0">
                            <columnConstraints>
                                <ColumnConstraints halignment="CENTER" hgrow="SOMETIMES" maxWidth="163.0" minWidth="10.0" prefWidth="146.0" />
                                <ColumnConstraints halignment="CENTER" hgrow="SOMETIMES" maxWidth="222.0" minWidth="10.0" prefWidth="170.0" />
                                <ColumnConstraints hgrow="SOMETIMES" maxWidth="184.0" minWidth="10.0" prefWidth="20.0" />
                            </columnConstraints>
                            <rowConstraints>
                                <RowConstraints maxHeight="176.0" minHeight="2.0" prefHeight="58.0" vgrow="SOMETIMES" />
                                <RowConstraints maxHeight="427.0" minHeight="0.0" prefHeight="52.0" vgrow="SOMETIMES" />
                                <RowConstraints maxHeight="489.0" minHeight="0.0" prefHeight="52.0" vgrow="SOMETIMES" />
                                <RowConstraints maxHeight="489.0" minHeight="0.0" prefHeight="73.0" vgrow="SOMETIMES" />
                                <RowConstraints maxHeight="489.0" minHeight="10.0" prefHeight="65.0" vgrow="SOMETIMES" />
                                <RowConstraints maxHeight="489.0" minHeight="10.0" prefHeight="83.0" vgrow="SOMETIMES" />
                        <RowConstraints maxHeight="489.0" minHeight="10.0" prefHeight="25.0" vgrow="SOMETIMES" />
                        <RowConstraints maxHeight="489.0" minHeight="10.0" prefHeight="27.0" vgrow="SOMETIMES" />
                        <RowConstraints maxHeight="489.0" minHeight="10.0" prefHeight="48.0" vgrow="SOMETIMES" />
                                <RowConstraints maxHeight="489.0" minHeight="10.0" prefHeight="55.0" vgrow="SOMETIMES" />
                        <RowConstraints maxHeight="489.0" minHeight="10.0" prefHeight="36.0" vgrow="SOMETIMES" />
                            </rowConstraints>
                            <children>
                                <Label alignment="CENTER" prefHeight="32.0" prefWidth="122.0" text="Название" />
                                <Label alignment="CENTER" prefHeight="32.0" prefWidth="144.0" text="Количество просмотров" GridPane.rowIndex="1" />
                                <Label alignment="CENTER" prefHeight="32.0" prefWidth="122.0" text="Продолжительность" GridPane.rowIndex="2" />
                                <Label alignment="CENTER" prefHeight="32.0" prefWidth="122.0" text="Время начала" GridPane.rowIndex="3" />
                                <Label alignment="CENTER" prefHeight="32.0" prefWidth="122.0" text="Состояние" GridPane.rowIndex="4" />
                                <TextField text="Нет названия" GridPane.columnIndex="1" />
                                <TextField text="0" GridPane.columnIndex="1" GridPane.rowIndex="1" />
                                <TextField text="Нет продолжительности" GridPane.columnIndex="1" GridPane.rowIndex="2" />
                                <TextField text="Нет времени" GridPane.columnIndex="1" GridPane.rowIndex="3" />
                                <Button mnemonicParsing="false" prefHeight="39.0" prefWidth="98.0" text="Добавить" GridPane.rowIndex="5">
                                    <font>
                                        <Font size="14.0" />
                                    </font>
                                </Button>
                                <Button mnemonicParsing="false" prefHeight="39.0" prefWidth="98.0" text="Сохранить" GridPane.columnIndex="1" GridPane.rowIndex="5">
                                    <font>
                                        <Font size="14.0" />
                                    </font>
                                </Button>
                                <Button mnemonicParsing="false" prefHeight="39.0" prefWidth="98.0" text="Старт" GridPane.columnIndex="1" GridPane.rowIndex="9">
                                    <font>
                                        <Font size="14.0" />
                                    </font>
                                </Button>


                                <ChoiceBox prefHeight="25.0" prefWidth="139.0" value="не начато" GridPane.columnIndex="1" GridPane.rowIndex="4">
                                    <items>
                                        <FXCollections fx:factory="observableArrayList">
                                            <String fx:value="не начато" />
                                            <String fx:value="активно" />
                                            <String fx:value="завершено" />
                                        </FXCollections>
                                    </items>
                                </ChoiceBox>
                        <ProgressBar prefHeight="18.0" prefWidth="322.0" progress="0.0" GridPane.columnSpan="2" GridPane.rowIndex="8" />
                        <Label alignment="CENTER" prefHeight="32.0" prefWidth="122.0" text="Прогресс" GridPane.columnSpan="2" GridPane.rowIndex="7" />
                        <Separator prefWidth="200.0" GridPane.columnSpan="3" GridPane.rowIndex="6" />
                            </children>
                        </GridPane>
                    </children>
                </HBox>
            </content>
        </Tab>
      
    </tabs>


    <stylesheets>
        <URL value="@myform.css" />
    </stylesheets>
</TabPane>



REPRODUCIBILITY :
This bug can be reproduced always.