osc 5577: `busy`, and a sidebar that shows it (busy_mark)

A status is a line of text, and a line of text cannot say whether it is still
true. "building 3/7" reads exactly the same while it builds and ten minutes
after it stopped, which is the one thing somebody reading it from another tab
wants to know -- and the only workaround available to a program was to write
the answer into the words and keep rewriting them.

So `busy`:

    printf '\033]5577;1;status;make test\033\\'
    printf '\033]5577;1;busy;1\033\\'
    ...
    printf '\033]5577;1;busy;0\033\\'

A flag of its own rather than a field of `status`, because a status is the
*whole* payload after the verb -- `status;a;b;c` has always been the text
"a;b;c" -- so anything added to that line would change what every existing
sender means, including the five pi extensions D1 promises byte compatibility
to. Orthogonal, so a program can leave one line up and toggle the other;
anything but 0/false/no/off is on; `clear` takes both; and a pane whose program
has gone is never busy, whatever it last claimed, because a spinner turning
under a dead tab is a lie you keep looking at.

In a sidebar the row turns `tab_status_busy` -- the theme's accent, derived
like the other status colours, so no theme needs a line -- and takes a spinner
in the column `tab_bar_padding` was already holding blank, so nothing reflows
when a program starts or stops working. `busy_mark` is the frames, separated by
spaces: one frame is a static mark and costs nothing, several make it turn, and
only then does the session keep a frame clock -- via the same `animating` flag
an animated shader uses, so the cost lands only while such a row is actually
composed, and `""` drops the mark entirely and leaves the colour to say it.
The frame is a pure function of the clock, like every other pulse here, so two
clients attached to one session show the same one.

`panes` now reports `status` and `busy`, which is the scriptable half of the
same fact: what a pane says about itself, without reading its screen.

1 files changed, 13 insertions, 0 deletions

diff --git a/docs/scripting.md b/docs/scripting.md
index 048a7fa..9971bc5 100644
--- a/docs/scripting.md
+++ b/docs/scripting.md
@@ -93,6 +93,8 @@ By printing an escape sequence. No plugin, no config:
 
 ```bash
 printf '\033]5577;1;status;building 3/7\033\\'
+printf '\033]5577;1;busy;1\033\\'     # ...and it is still going
+printf '\033]5577;1;busy;0\033\\'     # ...and now it is not
 printf '\033]5577;1;buttons;approve:Approve;cancel:Cancel\033\\'
 # clicking [Approve] arrives on the program's stdin as:
 #   \033]5577;1;click;approve\033\\
@@ -108,6 +110,17 @@ the pane that said it. A status is not decoration on your own frame; it is how
 a pane reports progress to somebody working elsewhere, which is a reason to
 keep it current and `clear` it when the work is done.
 
+`busy` is the one thing a line of text cannot say about itself: `make test` and
+`make test` are the same words whether it is running or finished. It is a flag
+of its own rather than a field of `status`, because a status is the *whole*
+payload after the verb — `status;a;b;c` is the text `a;b;c` — so anything added
+to that line would change what every sender already means. In a sidebar a busy
+status gets the spinner (`busy_mark`) and `tab_status_busy`, which is how
+"working" and "done" tell themselves apart from another tab without reading the
+words. Anything but `0`, `false`, `no` or `off` turns it on; `clear` turns it
+off along with the text, and so does the program exiting — a status left behind
+by something that is gone describes the past, whatever it last claimed.
+
 `purpose` is the other verb: `printf '\033]5577;1;purpose;logs\033\\'`. A purpose
 declared by a layout or the control API wins and cannot be overwritten this way.