When you scroll to a price that is far enough from the current market value that the current market value would normally scroll off the screen, the system prevents that from happening.
The line across DOMTrader ladder (in this case, under 129130) indicates that the market window is above the selected price. Every time the current market price changes, the market window re-centers the market price row, so that it is always displayed.
The market window is two rows by default. To change the size of the window, drag and drop the market window line (cursor becomes a double arrow), as shown in this image:
If the best bid and best ask are both displayed on DOMTrader and market movement results in DOMTrader not being to accommodate both (too great a spread), then an indicator is displayed between the two. It lets you know that there are prices missing, so that you can see both best bid and ask on the ladder. Notice the gap in this image:
