Files, open beside the conversation

A third side panel, built the way the terminal is and filled the way the
inspector is: tabs holding open files. Project files over SFTP in an agent
chat; notes, skills, knowledge documents, this chat's text attachments and its
own scratch document everywhere. Read with pygments, edited in a plain
textarea, saved with a conflict check.

A bug found on the way in, and the reason this needed its own read path.
`ssh.read_file` ends in `clean_output`, which strips ANSI escapes and decodes
with errors="replace" -- right for the output of a command, and fatal for an
editor: open a file containing an escape byte, press Save, and you have
silently rewritten it with the escapes gone and every undecodable byte replaced
by U+FFFD. `read_text`/`write_text` decode strictly, report binary rather than
mangling it, carry an mtime:size token for a file that moved underneath, and
refuse an oversize write rather than truncating -- `write_file` truncates
because a model is told how many bytes it wrote, and somebody pressing Save is
not. The model-facing pair is untouched: what it returns is a contract a model
has been shown. A truncated read opens read-only for the mirror-image reason.

Six sources go through one dispatch table, for the reason tool_labels.py is a
table: six independently written permission checks is how one ends up written
slightly differently, and that failure looks like editing somebody else's note.

A save on a project file bypasses agent/policy.py, which makes it the fourth
documented exception to "the modes do not govern the keyboard" and the first
that writes. Same argument as the terminal panel -- whoever owns the credential
could write the file with scp -- but the consequence is larger and is now said
out loud rather than left to be inferred.

The model opens tabs from the file tools it was already calling, so no new
schema and no tokens. It never brings one to the front: an agent reads forty
files in a long reply, and taking the screen each time would drag somebody
through all of them and lose any edit in progress. Only the strip is streamed,
guarded on truthiness so the frame can never blank itself -- an empty one would
close every open tab, the approval card you could press twice with the sign
reversed. Both halves are settled on the server, which is why canvas.js needs
no guard against a swap at all.

No vendored editor. CodeMirror 6 needs a bundler, which is hard rule 1;
CodeMirror 5 would be a larger payload than xterm on every page, and xterm is
the one heavy dependency precisely because it loads only where it can be used.
So: server-rendered highlighting for reading, a textarea for writing, and the
panel says there is no colour while you type rather than pretending.

Also here: a scratch document per chat, with `scratch_write` at RISK_READ on
plan_update's argument, and a test pinning the three numbers that decide a
panel's width -- LAYOUT_BOUNDS drops an unknown variable silently, so a panel
missing from it has a drag handle that works and forgets.

Driven under a DOM stub and against the running application.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/web/static/js/canvas.js

@@ -0,0 +1,111 @@
+/*
+  The canvas panel's small amount of behaviour.
+
+  Almost all of it is htmx: the tabs post, the save posts, the panel swaps.
+  Three things need JavaScript, and only three.
+
+  1. Tab inserts a tab character instead of leaving the field. Without it the
+     one editor affordance whose absence is genuinely maddening is missing.
+  2. The editor tracks whether it has been changed, so the tab shows a dot and
+     so leaving the page with unsaved work warns.
+  3. Opening the panel scrolls the active tab into view, since the strip
+     scrolls sideways and the tab in front may be off the end of it.
+
+  What is *not* here is any guard against a swap taking the editor away. A
+  model opening a file sends the tab strip and nothing else, and does not move
+  the active tab -- both settled on the server, where they cannot be lost to a
+  race.
+
+  Nothing here ever assigns innerHTML from a fetch: every swap is htmx's, and
+  the content is a file off somebody else's disk.
+*/
+(function () {
+  "use strict";
+
+  var panel = null;
+  /* Whether the editor has been changed since it was last rendered. Module
+     state rather than a data attribute, because the element it describes is
+     replaced by every swap and an attribute would go with it. */
+  var dirty = false;
+
+  function markDirty(on) {
+    dirty = !!on;
+    var dot = panel && panel.querySelector(".canvas__tab.is-active [data-canvas-dirty]");
+    if (dot) dot.hidden = !dirty;
+  }
+
+  /* --- Typing ------------------------------------------------------------- */
+  function onInput(event) {
+    if (event.target && event.target.matches("[data-canvas-editor]")) markDirty(true);
+  }
+
+  function onKeydown(event) {
+    var box = event.target;
+    if (!box || !box.matches || !box.matches("[data-canvas-editor]")) return;
+    if (event.key !== "Tab" || event.ctrlKey || event.altKey || event.metaKey) return;
+    /* Shift+Tab still leaves the field, which is the only way out of it for
+       somebody using the keyboard. */
+    if (event.shiftKey) return;
+
+    event.preventDefault();
+    var start = box.selectionStart;
+    var end = box.selectionEnd;
+    box.value = box.value.slice(0, start) + "\t" + box.value.slice(end);
+    box.selectionStart = box.selectionEnd = start + 1;
+    markDirty(true);
+  }
+
+  /* --- Swaps -------------------------------------------------------------- */
+  /* There is deliberately nothing here guarding the editor against a swap.
+     A model opening a file sends the tab *strip* and nothing else -- the body
+     is never pushed -- and `canvas.open_tab` does not move the active tab for
+     a model, so the file in front and the field being typed in both stay put.
+     That is settled on the server, where it cannot be lost to a race. */
+  function onAfterSwap(event) {
+    if (!panel || !event.target || !panel.contains(event.target)) return;
+    /* The server has just rendered what is stored, so nothing is unsaved until
+       somebody types again. */
+    markDirty(false);
+    showActiveTab();
+  }
+
+  function showActiveTab() {
+    var active = panel && panel.querySelector(".canvas__tab.is-active");
+    if (active && active.scrollIntoView) {
+      active.scrollIntoView({ block: "nearest", inline: "nearest" });
+    }
+  }
+
+  /* --- Leaving with unsaved work ------------------------------------------ */
+  function onBeforeUnload(event) {
+    if (!dirty) return;
+    event.preventDefault();
+    /* The browser shows its own wording; returning a string is what makes older
+       ones show anything at all. */
+    event.returnValue = "";
+    return "";
+  }
+
+  /* --- Wiring ------------------------------------------------------------- */
+  function start() {
+    panel = document.querySelector("[data-canvas]");
+    if (!panel) return;
+
+    panel.addEventListener("input", onInput);
+    panel.addEventListener("keydown", onKeydown);
+    panel.addEventListener("lembas:toggle", function (event) {
+      if (event.detail && event.detail.open) showActiveTab();
+    });
+
+    /* On document, not on the panel: htmx fires these on the element being
+       swapped, and by the time afterSwap runs the old node is gone. */
+    document.body.addEventListener("htmx:afterSwap", onAfterSwap);
+    window.addEventListener("beforeunload", onBeforeUnload);
+  }
+
+  if (document.readyState === "loading") {
+    document.addEventListener("DOMContentLoaded", start);
+  } else {
+    start();
+  }
+})();

src/lembas/web/static/js/commands.js

@@ -41,6 +41,7 @@
     { keys: "Alt + M", what: "Dictate" },
     { keys: "Alt + R", what: "Read the last reply aloud" },
     { keys: "Alt + 1 … 4", what: "Manual, Edit, Auto, Plan" },
+    { keys: "Alt + E", what: "Canvas" },
     { keys: "Alt + T", what: "Terminal" },
     { keys: "Alt + I", what: "Inspector" },
     { keys: "Alt + B", what: "Sidebar" },
@@ -135,6 +136,15 @@
       when: function () { return !!chat() && isAgent(); },
       run: function () { reindex(); }
     },
+    {
+      name: "canvas",
+      summary: "Show or hide the canvas",
+      /* Gated, like the other two panels. An ungated command on a page with no
+         panel does not merely fail -- it stops being a command, and the message
+         is sent as written. */
+      when: function () { return !!el("#canvas"); },
+      run: function () { toggle("#canvas", "side"); }
+    },
     {
       name: "terminal",
       summary: "Show or hide the terminal",
@@ -526,6 +536,13 @@
       return;
     }
 
+    /* E for editor, not C: Ctrl/Cmd+C is too near for comfort, and Alt+D is
+       the address bar in two browsers -- a shortcut the browser wins looks
+       broken. */
+    if (event.code === "KeyE" && el("#canvas")) {
+      event.preventDefault();
+      return toggle("#canvas", "side");
+    }
     if (event.code === "KeyT" && el("#terminal")) {
       event.preventDefault();
       return toggle("#terminal", "side");