Description Chester Kollschen 2013-04-10 19:30:52 EDT

The described problem occurs on a 32 bit machine running Debian GNU/Linux Wheezy.

I use the SWT as a stand-alone library in a JavaSE project that displays OpenGL graphics in a GLCanvas using LWJGL.

The GLCanvas is constructed once and then drawn to repeatedly.

The GLCanvas is constructed with the SWT.NO_BACKGROUND style bit so that SWT won't clear/fill the widget's background in the drawing process.

In the GLCanvas' PaintListener, OpenGL calls are used to draw the scene, followed by a call to GLCanvas.swapBuffers to make it visible.

Up to SWT 3.8.0, everything works fine. Starting with SWT 3.8.1, the scene flickers as if the SWT.NO_BACKGROUND style bit was not set.

I have diff'ed the source code folders of SWT 3.8.0 and SWT 3.8.1 and stumbled upon one change that may have something to do with this.

In the file <swt-sources>/org/eclipse/swt/widgets/Composite.java, method checkStyle(int):

    static int checkStyle (int style) {
        style &= ~SWT.TRANSPARENT;
        return style;
    }

In SWT 3.8.1, this was changed to:

    static int checkStyle (int style) {
        if (OS.INIT_CAIRO) {
            style &= ~SWT.NO_BACKGROUND;
	}
        style &= ~SWT.TRANSPARENT;
        return style;
    }

Both OS.INIT_CAIRO and OS.USE_CAIRO are initialize to true on my system. Since this method is called in the constructor of Canvas (and thus GLCanvas), this might be the cause of the troubles as well as a good starting point for investigation.

As a workaround, the system property "org.eclipse.swt.internal.gtk.useCairo" can be set to "false", which initializes OS.INIT_CAIRO to false, leaving the SWT.NO_BACKGROUND style bit intact and thus prevents flickering.