GWT FocusPanel setProperty("left",xx") does not work

Question

I am implementing "drag" applying "set left".

Here are my code:

scrollPanel.setWidget(focusPanel);//add a focusPanel to the existing scrollPanel
focusPanel.setWidget(flowPanel); //add a flowPanelto the existing focusPanel
flowPanel.add(image) // add a image to  the flowPanel

scrollPanel.setPixelSize(scrollPanle_width,scrollPanle_height) focusPanel.setPixelSize(image_width, image__height)

Enlarge the image, so image size is larger than scrollPanel size

Try to drag the image to be viewed inside scrollPanel,

(1)Working way

flowPanel.getElement().getStyle().setPropertyPx("left",  offset );

When drag, image could be moved.

(2)Not working

focusPanel.getElement().getStyle().setPropertyPx("left",  offset );

When drag, image could not be moved. Also try this and no luck Widget focusPanelWidget = scrollPanel.getWidget(); focusPanelWidget.getElement().getStyle().setPropertyPx("left", offset );

So Any one could help to see why "set left" does not work for focusPanel? Do appreciate any help!

Many thanks!