In the Tabs panel, a stop dragged on the ruler with Ctrl held (Command on
macOS) moves every stop by the same distance, set in one text.tabs.set and
one undo step. The stops keep their order, alignment, leaders and Align On,
and the ruler draws them at their new positions during the drag.
Ctrl is read when the drag starts, as the Left Indent marker reads it. A drag
that would take the leftmost stop below 0 leaves it at 0, and one that would
take the rightmost past 100,000 pt leaves it at 100,000 pt, keeping the
spacing of the others. A Ctrl-drag released off the ruler deletes only the
dragged stop, as a plain drag does. A plain drag still moves one stop.