<p>You can retain any selection made in the Linedefs, Sectors and Things modes while moving from 2D editing to Visual mode (and back). To do this, hold <strong>Shift</strong> while going to/from Visual mode (that will be <strong>Shift-Q</strong> by default). You can also invert this behavior by enabling<strong> Preferences -> Editing ->"Synchronize selection between Visual and Classic modes"</strong> setting. When this setting is enabled, holding <strong>Shift</strong> will ignore selection.</p>