Shortly, the mechanism of repainting of a JLF is the following. Once a JLF's child component is requesting repaint, an appropriate repaint runnable is scheduled by the RepaintManager (RM). The runnable is then gets dispatched by the RM which calls the paint() method of the root component, that is the JLF. JLF overrides this method in the way that after all the painting is done (super.paint) it initiates a pixel bits transfer to the host application (e.g. SwingNode). In case of JViewport, when it works in the default BLIT scroll mode, scrolling of the JViewport doesn't lead to a repaint runnable being dispatched. Instead, JViewport immediately repaints its content (via blitting + repainting a dirty area) and then tells the RM there's nothing to repaint (so the runnable scheduled is just skipped). As the result, the JLF doesn't get any notification of the update.
As a workaround to this problem, JViewport had been forcibly switched to the BACKINGSTORE scroll mode, in which case it passes the whole repainting cycle.
This JIRA issue is aimed at implementing a mechanism which would let the JViewport work in its default BLIT scroll mode, which has better performance characteristics comparing to the BACKINGSTORE mode.