sl0ppty: three ways back from a painted pane

    printf '\033]5577;1;shader;\033\\'                  # from the program
    sl0ppty cmd '{"cmd":"clear-shaders","id":0}'   # from outside it
    bind "z" "clear-shaders"                       # from the keyboard

`app_clear_pane_shaders` empties both of a pane's chains and says whether
there was anything there, which is what lets a key answer "shaders cleared"
or "no shaders on this pane" instead of maybe having done something.

In-band it is not a new verb: naming no rect means all of them. `shader;`
sits under `shader;chrome;` and `shader;content;` as the degenerate case of
"which rect", so a program puts a pane back in one exchange and cannot leave
it half done. `contrib/shader-repl`'s `:both` now uses that rather than two.

**None of the three is gated on `in_band_shaders`.** A chain outlives the
setting that allowed it -- paint a pane, then turn the setting off, and the
paint is still there, because a chain belongs to the pane and not to the file
(there is a test that reloads with it off and clears anyway). A way out that
the setting can take away is not a way out. The operator path matters for the
same reason: the program that made a pane unreadable is not always in a state
to put it back.

They clear what the *pane* set. The config's chains and the session's own
dimming are not this pane's doing and are derived every frame anyway.

The action is bound to nothing, like `scroll-up`: it is in the palette, which
is what the palette is for, and the docs say to bind it if you leave the
setting on. Which is where a second bug turned up -- the palette truncates a
label at 26 characters, so "undo this pane's own shaders" read as "undo this
pane's own shade" on screen. Renamed to fit, and `tests/test_palette.py` now
checks every label against the column, since nothing failed for that: it just
looked like a typo in front of someone.

3 files changed, 26 insertions, 4 deletions

diff --git a/docs/keys.md b/docs/keys.md
index 66163b5..a5befad 100644
--- a/docs/keys.md
+++ b/docs/keys.md
@@ -95,7 +95,7 @@ being bound at all.
 
 | group | actions |
 |---|---|
-| panes | `split-cols` `split-rows` `close-pane` `rerun` `zoom` `minimize` `rotate-layout` |
+| panes | `split-cols` `split-rows` `close-pane` `rerun` `zoom` `minimize` `rotate-layout` `clear-shaders` |
 | focus | `focus-left` `focus-right` `focus-up` `focus-down` `focus-next` `finder` |
 | size | `resize-left` `resize-right` `resize-up` `resize-down` `equalize` |
 | tabs | `new-tab` `next-tab` `prev-tab` `select-tab-1` … `select-tab-9` |
@@ -103,4 +103,7 @@ being bound at all.
 | session | `palette` `help` `edit-config` `detach` `quit` `literal-prefix` |
 
 `scroll-up` and `scroll-down` have no default key — the wheel does that job —
-which is exactly the case the palette exists for.
+which is exactly the case the palette exists for. So does `clear-shaders`, which
+undoes whatever the program in a pane painted on it
+([shaders](shaders.md#prototyping-in-a-pane)); bind it if you leave
+`in_band_shaders` on.
diff --git a/docs/scripting.md b/docs/scripting.md
index 298ae17..0f398ff 100644
--- a/docs/scripting.md
+++ b/docs/scripting.md
@@ -29,6 +29,7 @@ Panes and tabs are addressed by **id**, so a background tab is scriptable.
 | `split` | `dir:"cols"\|"rows"`, `id` for which pane to split |
 | `focus` | `id` |
 | `close` `rerun` | `id`, or 0 for the focused pane |
+| `clear-shaders` | `id`, or 0 for the focused pane; answers `cleared:0\|1` — the way back from a pane that painted itself unreadable |
 | `new-tab` `select-tab` `close-tab` `move-tab` `set-name` | tabs, by `id` or `index` |
 | `set-purpose` | `target:"pane"\|"tab"`, `id`, `purpose` |
 | `dump-layout` `apply-layout` | see [layouts](layouts.md) |
@@ -61,7 +62,10 @@ declared by a layout or the control API wins and cannot be overwritten this way.
 `shader` is the third, and the only one the session can refuse: it sets the
 shader chain for the pane that asked, in the config's own syntax, and needs
 `in_band_shaders true` because a program restyling your session is a hazard
-before it is a convenience. See [shaders](shaders.md#prototyping-in-a-pane).
+before it is a convenience. `shader;` with no rect named clears both of that
+pane's chains, which is never refused -- and `clear-shaders` above is the same
+thing from outside, for a program that will not do it itself.
+See [shaders](shaders.md#prototyping-in-a-pane).
 
 Anything the session sends *back* to a program ends its verb in `-reply` --
 `hello-reply`, `shader-reply` -- and no request verb may. A pane that echoes what
diff --git a/docs/shaders.md b/docs/shaders.md
index 63801db..bb0221d 100644
--- a/docs/shaders.md
+++ b/docs/shaders.md
@@ -96,7 +96,8 @@ set the chains for the pane it is running in, in the same syntax the config uses
 ```bash
 printf '\033]5577;1;shader;chrome;tint color="#ff5fd7" amount="abs(t / 8 %% 510 - 255)"\033\\'
 printf '\033]5577;1;shader;content;dim amount=90\033\\'
-printf '\033]5577;1;shader;chrome;\033\\'   # nothing: back to normal
+printf '\033]5577;1;shader;chrome;\033\\'   # that rect: back to normal
+printf '\033]5577;1;shader;\033\\'          # no rect named, so both of them
 ```
 
 The field after `shader` is which rect (`content` or `chrome`), and the rest of
@@ -105,6 +106,20 @@ on one line. The session answers on the program's stdin, `\033]5577;1;shader-rep
 or `shader-reply;error;bad amount for tint: ...`, so a typo says so instead of
 looking like a shader that does nothing.
 
+**Three ways to take it all back**, because the program that painted a pane is
+not always in a state to put it back:
+
+| | |
+|---|---|
+| `printf '\033]5577;1;shader;\033\\'` | from the program, one exchange, both rects |
+| `sl0ppty cmd '{"cmd":"clear-shaders","id":3}'` | from outside; `id` 0 or absent means the focused pane |
+| the `clear-shaders` action | on a key you bind, or from the palette |
+
+None of them is gated on `in_band_shaders`. A chain can outlive the setting that
+allowed it -- paint a pane, then turn the setting off -- and a way out that the
+setting can take away is not one. They clear what the *pane* set: the config's own
+chains and the session's own dimming are not this pane's doing.
+
 `contrib/shader-repl` is that loop with a prompt on it: type a chain, see the
 pane change, and `:paste` prints what you have as a `shaders { }` block for your
 config. What you prototype and what you paste are parsed by the same code, which