When working on dense designs, you may find that you need to start a "Frame Select" operation without doing the normal "mouse down" action that would probably start dragging something on your design. This FAQ describes how to program your shortcut keys to do this.
From the Settings menu, select Customimse, and choose the Shortcut Keys tab. In the Category drop-down list, choose Interaction. In the list of Commands, choose "Frame Select From Cursor". Now you can assign a shortcut key to that command, and close the Customise dialog.
Once you have assigned your shortcut key, you can move the cursor to the place where you want to start the Frame, press your new shortcut key, then move the cursor to the other corner and click the left mouse button (or press Space) to finish the Frame.