Allow offscreen mine control selection

When selecting an area for the mine control tower, many times my mouse will go offscreen and my selection will be ignored. I then have to re-open the mine control tower and make a new selection. If the mouse pointer goes offscreen when making a selection, mark the last known location of the mouse as the bounds of the selection. Another approach is to scroll the window to where ever the max boundary is.

It would also be helpful if the Mine Control Tower dialog would stay open unless explicitly closed.