Drag cards out into columns, and columns among and into columns

The user wants to drag a card out between columns to make a new column,
and to drag columns to reorder them or put them inside other columns.

Their pick for where: the row of titles, the gaps between columns and
the space past the last one are among the columns. Something dragged
there becomes a column, before the column under the mouse on its left
half and after it on its right half, after the one to the left in a gap.
Anywhere else it goes into the column under the mouse by the card rules,
so a column dragged down into another one becomes a card, its own cards
its block. A column's title no longer means "its first card"; the top
half of its first card does that. The root's column stays first and
can't be dragged (the user's pick): it says so.

The board holds still around a drag. Besides the kept widths, the room a
dragged column took stays empty while it isn't a column (Drag.room):
without it, the columns to its right slid left under the mouse each
time it stopped being one, and the card landed a column too far.
Dropping it back in that room makes it that column again.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

DESIGN.md

@@ -362,13 +362,22 @@ lines.
   at its end; a column's own node trades places with the column beside
   (the root's stays first). Enter is done, one undo step. Every move goes
   through the tree.
-- A drag moves the card live, as the canvas does (the user's pick): it goes
-  through the board as the mouse goes, by the row the mouse is on in the
-  column under it. On a card's top half it goes before that card; on its
+- A drag moves a card, or a column by its title, live, as the canvas does
+  (the user's pick); the whole drag is one undo step. The row of titles,
+  the gaps between columns and the space past the last one are among the
+  columns (the user's pick): there it becomes a column, before the column
+  under the mouse on its left half, after it on its right half, after the
+  one to the left in a gap. Anywhere else it goes into the column under
+  the mouse, as a card, by the row: on a card's top half before it; on its
   bottom half after it, or into its block as the first card; on a block's
-  foot, after the block; on a column's title, first; below the cards,
-  last. The whole drag is one undo step. The columns keep their widths
-  while a card is dragged, so the one under the mouse stays put.
+  foot, after the block; below the cards, last. So a card dragged out
+  becomes a column, and a column dragged into another one becomes a card,
+  its cards its block. The root's column stays first and can't be dragged
+  (the user's pick: it's the document's root).
+- The board holds still around a drag: the columns keep the widths they
+  had, and the room a dragged column took stays empty while it isn't one
+  (dropping it there makes it that column again). Otherwise the columns
+  beside would slide under the mouse, and the card would chase them.
 - In visual mode the board selects in reading order, as the outline does.
 
 **Views, panes and popups** (settled in stage 7)
@@ -568,9 +577,11 @@ pans up and down, and sideways with a wheel that tilts, a touchpad, or the
 wheel with Shift or Alt held (Alt for terminals that keep Shift for selecting
 text), Ctrl+drag selects a rectangle, Ctrl+click adds or removes one node,
 right-click opens a menu built from the same actions as the keys. Zoom is on
-the `+` and `-` keys. On the board, dragging a card moves it through the
-tree as the mouse goes: before or after the card under it (by which half),
-into a block, out at its foot, or to a column's top or end.
+the `+` and `-` keys. On the board, dragging a card, or a column by its
+title, moves it through the tree as the mouse goes: before or after the
+card under it (by which half), into a block, out at its foot, to a
+column's end, or among the columns (on the titles' row or in a gap) as a
+column of its own.
 
 ## Modes and keys