Steps to reproduce:
1. Run any JavaFX 8 application, example simple "HelloWorld".
2. Click to button "Minimize".
3. Wait any time (30 sec).
4. Restore app window.
5. Content of app window is black.
6. Or repeatedly click to app icon in taskbar to restore/minimize app window.
7. Content of app window is black.
8. App window content is visible only when the mouse is hovered the window.

This bug is does not occur always.

Test app:

package test;

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class JavaFXApplicationTest extends Application {
    
    @Override
    public void start(Stage primaryStage) {
        Button btn = new Button();
        btn.setText("Say 'Hello World'");
        btn.setOnAction((ActionEvent event) -> {
            System.out.println("Hello World!");
        });
        
        StackPane root = new StackPane();
        root.getChildren().add(btn);
        
        Scene scene = new Scene(root, 300, 250);
        
        primaryStage.setTitle("Hello World!");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

1. Run with: java -showversion  -Dprism.order=sw -Dprism.verbose=true test.JavaFXApplicationTest
Result: bug is present

java version "1.8.0-ea"
Java(TM) SE Runtime Environment (build 1.8.0-ea-b120)
Java HotSpot(TM) 64-Bit Server VM (build 25.0-b62, mixed mode)

Prism pipeline init order: sw
Using platform text rasterizer
Using native-based Pisces rasterizer
Using dirty region optimizations
Not using texture mask for primitives
Not forcing power of 2 sizes for textures
Using hardware CLAMP_TO_ZERO mode
Opting in for HiDPI pixel scaling
*** Fallback to Prism SW pipeline
Prism pipeline name = com.sun.prism.sw.SWPipeline
(X) Got class = class com.sun.prism.sw.SWPipeline
Initialized prism pipeline: com.sun.prism.sw.SWPipeline
 vsync: true vpipe: false
Loading Prism common native library ...
        succeeded.

2. Run with: java -showversion -Dprism.verbose=true test.JavaFXApplicationTest
Result: bug is present

java version "1.8.0-ea"
Java(TM) SE Runtime Environment (build 1.8.0-ea-b120)
Java HotSpot(TM) 64-Bit Server VM (build 25.0-b62, mixed mode)

Prism pipeline init order: d3d sw
Using platform text rasterizer
Using native-based Pisces rasterizer
Using dirty region optimizations
Not using texture mask for primitives
Not forcing power of 2 sizes for textures
Using hardware CLAMP_TO_ZERO mode
Opting in for HiDPI pixel scaling
Prism pipeline name = com.sun.prism.d3d.D3DPipeline
Loading D3D native library ...
        succeeded.
D3DPipelineManager: Created D3D9 device
Direct3D initialization succeeded
(X) Got class = class com.sun.prism.d3d.D3DPipeline
Initialized prism pipeline: com.sun.prism.d3d.D3DPipeline
Maximum supported texture size: 8192
Maximum texture size clamped to 4096
OS Information:
        Windows 7 build 7601
D3D Driver Information:
        Intel(R) HD Graphics
        \\.\DISPLAY1
        Driver igdumd64.dll, version 9.17.10.3347
        Pixel Shader version 3.0
        Device : ven_8086, dev_0102, subsys_102517AA
        Max Multisamples supported: 4
 vsync: true vpipe: true
Loading Prism common native library ...
        succeeded.

Bug is present at both D3D Pipeline, SW Pipeline!!!