tabs: air before the new-tab button, and the option to do without it

The `+` is the one entry in the strip that does not mean "go here", and it
sits at the end of a list where every other entry does. Flush against them it
is what you hit when you meant the last tab -- most easily in a sidebar, where
the target is the full width of the row and the pointer only has to be one row
low. newtab_pad is the nothing in between: blank rows in a sidebar, blank
columns on top, one by default, which is enough to make the slip a miss.

It is paint rather than layout, like tab_bar_pad: the panes' rectangle is the
same at 0 as at 5, so tuning it repaints without reflowing anyone. And it is
given up before the button is -- a strip short of room drops the spacing one
cell at a time and still draws what the spacing was protecting, because
spacing must never cost you the thing it spaces.

newtab_button false removes the button outright, for people who make tabs from
the keyboard and would rather have the row (or the columns) back. Only the
place to click it goes: the chord and the palette entry still work. Setting
newtab_mark "" already did this by omission; this says it on purpose, and says
it in --dump-config.

1 files changed, 10 insertions, 1 deletions

diff --git a/docs/panes.md b/docs/panes.md
index f5fab23..1884b53 100644
--- a/docs/panes.md
+++ b/docs/panes.md
@@ -57,13 +57,22 @@ tab_bar_side "left"   // or "right"; "top" is the default
 tab_bar_width 18      // columns the sidebar takes
 tab_bar_pad 0         // blank rows above the first tab
 tab_bar_chrome true   // frame it like a pane
+newtab_button true    // the `+` at the end of the list
+newtab_pad 1          // ...and the air before it
 ```
 
 A **sidebar lists one tab per row** down the chosen edge, and the panes trade
 the strip's row for its columns. Everything the strip does travels with it:
 click to switch, double-click to rename in place, drag a pane onto a tab or
 onto the `+` — which, given a row of its own and the columns to spare, spells
-itself out as `+ new tab`. The pane count and the prefix badge move
+itself out as `+ new tab`. That button sits `newtab_pad` rows below the last
+tab (columns after it, in the top strip): it is the one row in the list that
+does something other than "go here", and flush against the tabs it is what you
+hit when you meant the last one. The air is paint, not layout — the panes keep
+their rectangle whatever it is, and it is given up before the button is when
+the strip runs out of room. Set `newtab_button false` to drop the button
+altogether; `new-tab` is still a [chord](keys.md) and a palette entry, and a
+pane can no longer be dropped onto it. The pane count and the prefix badge move
 to the sidebar's bottom rows; the [status line](config.md) along the bottom
 keeps the full width. A terminal too narrow to give up the columns falls back
 to the top row until it grows.