Opening an image that is larger than the available window space when the navigation bar is visible produces repetitive repaints of the image. Enlarging the main window so that the image and the navigation bar do not intersect makes the flickering to disappear.
This is actually not the navigation-bar. This is a race-condition when the scrollbars get visible and invisible continuously when having the resizing the image to fit the window.
Fixed in this commit > http://git.xfce.org/apps/ristretto/commit/?id=23c1c7206a9b423d8f53c7ca2aa8b980131bf894 From the commit-msg: Move update of the adjustment page_size to the queued_repaint callback function. This way the adjustments do not get their updates before the _upper limit is changed too. This fixes a problem where the image-viewer is updated too often, sometimes indefinately until the widget-size is changed again.