I'm using 2 monitors. On the 2nd one, I have the Delphi IDE, using the Classic undocked settings.
There is a white vertical line in the middle of the IDE cutting through the source code. I can't seems to find the option in the settings to remove it.
That line is the Code Editor's "Right margin". You can turn that on/off in the Display section of the Code Editor settings, or you can set how many characters from the left edge of the editor you want the line to appear (the default is 80 chars).
See Customizing the Code Editor for more details.