How to prevent vanishing of shapes in SWT Scrollable Composite - swt

Question

Im trying to create a little Telnet Client for a MUD. In this program i want to have a scrollable map. I already made this and it works pretty good. But as soon as i reach the borders it deletes the rectangles (painted with GC) outside the window. If i go back to them, the shapes are not there anymore.
Little Gif to show the problem:
https://gifyu.com/image/vhnq
Im scrolling with the the ScrolledComposite.setOrigin(Point p) function inside an async Runnable.
Canvas declaration + parents (room is an int with value 10):
Composite container = new Composite(mapShell, SWT.FILL);
container.setLayout(new GridLayout(2, false));
scrolledComposite = new ScrolledComposite(container, SWT.BORDER | SWT.H_SCROLL | SWT.V_SCROLL);
scrolledComposite.setVisible(true);
scrolledComposite.setAlwaysShowScrollBars(false);
scrolledComposite.setLayoutData(new GridData(SWT.FILL, SWT.CENTER, true, true, 1, 4));
int width = 10000, height = 10000;
Canvas mapCanvas = new Canvas(scrolledComposite, SWT.NONE);
mapCanvas.setBackground(mapShell.getDisplay().getSystemColor(SWT.COLOR_DARK_GRAY));
mapCanvas.setSize(width, height);
mapCanvas.setLocation(0, 0);
final ScrollBar hBar = scrolledComposite.getHorizontalBar();
final ScrollBar vBar = scrolledComposite.getVerticalBar();
hBar.setIncrement(room * 2);
vBar.setIncrement(room * 2);
gc = new GC(mapCanvas);
mapCanvas.setLayout(null);
scrolledComposite.setContent(mapCanvas);
scrolledComposite.setMinSize(new Point(300, 400));
Function to draw the rooms:
public void changeCurrentRoom(Room newRoom) {
mapShell.getDisplay().asyncExec(new Runnable() {
#Override
public void run() {
gc.setBackground(currentRoom.getColor());
gc.fillRectangle(currentRoom.getRectangle());
currentRoom = newRoom;
gc.setBackground(ACTIVE_ROOM_COLOR);
gc.fillRectangle(currentRoom.getRectangle());
}
});
mapShell.getDisplay().asyncExec(new Runnable() {
#Override
public void run() {
scrolledComposite.setOrigin(getScrollPosition(currentRoom));
roomText.setText(currentRoom.getName());
}
});
}
Thanks for your help!