A Rectangular selection can now be started even when a map element is already highlighted. This means that you can now use it to select sectors, which are completely surrounded by other sectors.
Hold Shift to enable additive selection, hold Ctrl to enable subtractive selection, hold Ctrl-Shift to intersect with an existing selection.
Hold Alt in Sectors mode to select things inside of selected sectors.
Current behaviour is indicated by the selection outline: additive selection uses the Highlight color, subtractive selection uses the inverted Highlight color.
The Rectangular selection mode in Sectors and Linedefs modes can be toggled between "Select inside" (classic behavior: only map elements which are fully inside selection will be affected) and "Select touching" (map elements which are partially inside selection will be also affected) using this button in the toolbar:
