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, 5 insertions, 4 deletions

diff --git a/include/config.h b/include/config.h
index a042141..e24817a 100644
--- a/include/config.h
+++ b/include/config.h
@@ -471,10 +471,11 @@ typedef struct {
   /* The fill under every tab that is not the active one, so a tab reads as a
    * plate rather than as words floating on the background -- and so the
    * active tab's fill reads as *this one* against the rest, which is a
-   * comparison it could not make against nothing. Derived a third of the way
-   * from the background to `tab_idle`, the tab's own label grey, so it stays
-   * in a theme's family without any theme naming it. Set it to the theme's
-   * `default_bg` for the old bare look. */
+   * comparison it could not make against nothing. Derived 43% of the way from
+   * the background to `tab_idle`, the tab's own label grey, so it stays in a
+   * theme's family without any theme naming it -- and lands at about the same
+   * 1.9:1 against the background whether that background is dark or light.
+   * Set it to the theme's `default_bg` for the bare look. */
   color_t tab_idle_bg;
   color_t prefix_fg, prefix_bg; /* the "C-a" badge */
   color_t tab_count;            /* how many panes in the session */