tabs: idle tabs you can actually read across the room

Seen next to the accent, the first plate was too polite to do the job it was
added for: a 30% lift off the background under labels in the same #45454a the
idle frames use. From another monitor the strip still read as one dark band
with a blue block in it.

Both ends move. tab_idle leaves the ambient chrome grey and becomes #8a8a95,
because a tab you are not in is a place to go and has to be legible at a
glance, where a dimmed pane border only has to be findable. The plate follows
it to 43%, which is not a taste constant but a contrast ratio: the same step
taken from a dark background upward and a light one downward lands at about
1.9:1 either way, so paper's #bdbebe on near-white and the default's #3b3b40
on black are the same amount of plate.

The six themes in contrib named their idle tab colour as their frame grey, so
each gets the same move in its own family -- 40% of the way from that grey to
the colour the theme writes text in -- and their plates follow by derivation:
amber #a67c26, mono #909090, paper #666a6e, phosphor #4a9c5d, slate #70789c,
sl0p the house #8a8a95. Nobody who named a colour loses it; the derivation
only ever fills in a theme's silence.

1 files changed, 7 insertions, 3 deletions

diff --git a/docs/panes.md b/docs/panes.md
index 087c873..6062280 100644
--- a/docs/panes.md
+++ b/docs/panes.md
@@ -127,9 +127,13 @@ terminal's own background alone. The top strip has one row and no room for any o
 why the sidebar exists.
 
 **Every tab is a plate, not a word on the background.** The one you are in is
-filled with the accent (`tab_active_bg`); the rest sit on `tab_idle_bg`, a
-third of the way from the theme's background to the grey it writes idle labels
-in. Two things come of the quiet fill: a tab has edges, so where one ends, how
+filled with the accent (`tab_active_bg`); the rest sit on `tab_idle_bg`, 43% of
+the way from the theme's background to `tab_idle`, the grey it writes their
+labels in — a fraction chosen as a contrast ratio, about 1.9:1 against the
+background whether the theme is dark or light. That label grey is brighter than
+the one on idle frames and titles, because a tab you are not in is a place to
+go and has to read from across the screen, where a dimmed border only has to be
+findable. Two things come of the quiet fill: a tab has edges, so where one ends, how
 far the row you are about to click reaches, and — in the sidebar especially —
 which rows are tabs rather than statuses are all visible; and the active fill
 has something to be brighter *than*, which on a strip of short labels is most