✨ hardware cursor positioning via text() caret

AGENTS.md

@@ -30,8 +30,10 @@ Do not include any agent marketing material (e.g. "Generated with...",
 
 - The renderer MUST NOT perform IO. It produces bytes; the caller writes them.
 
-- The renderer MUST NOT manage terminal state (alternate buffer, cursor
-  visibility, mouse reporting, keyboard protocol modes).
+- The renderer MUST NOT manage terminal state (alternate buffer, mouse
+  reporting, keyboard protocol modes). Cursor visibility and positioning are
+  renderer-managed only when a `caret` declaration is present on a `text()`
+  directive (see renderer-spec.md §7.6).
 
 - Each frame is a complete snapshot. The renderer carries no UI tree state
   between frames — only cell buffers for diffing.

examples/text-input/index.ts

@@ -0,0 +1,143 @@
+import { Buffer } from "node:buffer";
+import process from "node:process";
+import { each, ensure, main, until } from "effection";
+import {
+  close,
+  createTerm,
+  fixed,
+  grow,
+  type KeyEvent,
+  type Op,
+  open,
+  rgba,
+  text,
+} from "../../mod.ts";
+import { alternateBuffer, progressiveInput, settings } from "../../settings.ts";
+import { useInput } from "../use-input.ts";
+import { useStdin } from "../use-stdin.ts";
+
+const bg = rgba(20, 20, 30);
+const inputBg = rgba(35, 35, 50);
+const label = rgba(180, 180, 200);
+const hint = rgba(80, 80, 100);
+
+await main(function* () {
+  let { columns, rows } = terminalSize();
+
+  setRawMode(true);
+
+  let stdin = yield* useStdin();
+  let input = useInput(stdin);
+
+  let term = yield* until(createTerm({ width: columns, height: rows }));
+
+  let tty = settings(alternateBuffer(), progressiveInput(1));
+  writeStdout(tty.apply);
+
+  let value = "";
+  let caret = 0;
+
+  yield* ensure(() => {
+    setRawMode(false);
+    writeStdout(tty.revert);
+  });
+
+  let { output } = term.render(frame(value, caret));
+  writeStdout(output);
+
+  for (let event of yield* each(input)) {
+    if (event.type === "keydown") {
+      let key = event as KeyEvent;
+
+      if (key.ctrl && key.key === "c") {
+        break;
+      }
+
+      if (key.key === "Escape") {
+        break;
+      }
+
+      if (key.key === "ArrowLeft") {
+        if (caret > 0) {
+          caret--;
+        }
+      } else if (key.key === "ArrowRight") {
+        if (caret < [...value].length) {
+          caret++;
+        }
+      } else if (key.key === "Backspace") {
+        if (caret > 0) {
+          let chars = [...value];
+          chars.splice(caret - 1, 1);
+          value = chars.join("");
+          caret--;
+        }
+      } else if (
+        key.key.length === 1 &&
+        !key.ctrl &&
+        !key.alt
+      ) {
+        let chars = [...value];
+        chars.splice(caret, 0, key.key);
+        value = chars.join("");
+        caret++;
+      }
+
+      ({ output } = term.render(frame(value, caret)));
+      writeStdout(output);
+    }
+
+    yield* each.next();
+  }
+});
+
+function frame(value: string, caret: number): Op[] {
+  return [
+    open("root", {
+      layout: {
+        width: grow(),
+        height: grow(),
+        direction: "ttb",
+        padding: { left: 2, right: 2, top: 1, bottom: 1 },
+        gap: 1,
+      },
+      bg,
+    }),
+    open("label", { layout: { height: fixed(1) } }),
+    text("Name:", { color: label }),
+    close(),
+    open("input-box", {
+      layout: {
+        width: fixed(40),
+        height: fixed(1),
+        padding: { left: 1, right: 1 },
+      },
+      bg: inputBg,
+    }),
+    text(value, { color: label, caret }),
+    close(),
+    open("hint", { layout: { height: fixed(1) } }),
+    text("← → move  Backspace delete  Esc or Ctrl+C exit", { color: hint }),
+    close(),
+    close(),
+  ];
+}
+
+function terminalSize(): { columns: number; rows: number } {
+  return process.stdout.isTTY
+    ? {
+      columns: process.stdout.columns ?? 80,
+      rows: process.stdout.rows ?? 24,
+    }
+    : { columns: 80, rows: 24 };
+}
+
+function setRawMode(enabled: boolean): void {
+  if (process.stdin.isTTY && typeof process.stdin.setRawMode === "function") {
+    process.stdin.setRawMode(enabled);
+  }
+}
+
+function writeStdout(bytes: Uint8Array): void {
+  process.stdout.write(Buffer.from(bytes));
+}

ops.ts

@@ -315,6 +315,10 @@ export function pack(
         );
         o += 4;
 
+        // Caret offset (code points), or 0xFFFFFFFF when absent.
+        view.setUint32(o, op.caret ?? 0xFFFFFFFF, true);
+        o += 4;
+
         let str = encoder.encode(op.content);
         o = packString(view, str, o, end, "text content");
         break;
@@ -481,6 +485,7 @@ export interface Text {
   fontId?: number;
   wrap?: number;
   attrs?: number;
+  caret?: number;
 }
 
 interface Snapshot {
@@ -533,7 +538,7 @@ function packSize(ops: Op[]): number {
         break;
       }
       case OP_TEXT: {
-        n += 4 + 4 + 4 + 4; // opcode + color + bg + cfg
+        n += 4 + 4 + 4 + 4 + 4; // opcode + color + bg + cfg + caret
         n += 4 + Math.ceil(encoder.encode(op.content).length / 4) * 4; // string
         break;
       }

specs/renderer-spec.md

@@ -334,6 +334,75 @@ nests clip regions more deeply than the renderer can track:
   (see §12.3) before returning, so the caller can detect that some clipping was
   not applied.
 
+### 7.6 Hardware cursor visibility and positioning
+
+A `text()` directive MAY declare a `caret` property whose value is a
+non-negative integer code-point offset into the directive's `content`. `0` means
+"before the first code point"; `[...content].length` means "after the last."
+Offsets in between sit immediately before the cell where the corresponding code
+point would render.
+
+The cell where a declared caret sits is the cell at which the code point at the
+caret's offset would be drawn given the layout engine's text wrapping. For an
+offset `N`:
+
+- If `N < [...content].length`, the caret's cell is the display position of the
+  `N`-th code point (zero-indexed) within the rendered text.
+- If `N == [...content].length`, the caret's cell is one display position past
+  the last rendered code point: on the same wrapped line if there is room, or at
+  the start of the next line if the layout wraps at the end.
+- If `N > [...content].length` or `N < 0`, behavior is unspecified; callers must
+  keep offsets within bounds.
+
+When the layout engine consumes whitespace at a wrap boundary — dropping it from
+the rendered text so that neither wrapped line contains a display cell for those
+code points — the caret has no display position of its own for any offset that
+falls in that dropped run. In that case the caret's cell is the origin of the
+following wrapped line (the display position of the first code point on that
+line). This rule keeps the caret on-screen at the point where subsequent input
+will land, rather than orphaning it past the end of the previous line.
+
+When `content` is empty, the only in-range offset is `0`. In that case the
+caret's cell is the text element's origin — the cell at which the first code
+point of `content` would be drawn if `content` were a single space. This is a
+rendering commitment implied by the presence of a `caret` declaration, and the
+renderer must synthesize whatever minimal geometry is required to honor it. How
+this outcome is achieved is implementation-defined.
+
+Display position accounts for code points wider than one cell (CJK, fullwidth
+forms, some emoji): such code points occupy two cells, and the caret sits at the
+first cell of the pair. Cell widths are determined by the same measurement the
+renderer uses to lay out the text itself.
+
+The renderer manages the terminal's hardware cursor based on the presence of
+`caret:` declarations:
+
+- When the current frame contains one or more `caret:` declarations, the
+  rendered `output` MUST, when written to the terminal, leave the hardware
+  cursor visible at the cell where the first declared caret (in directive order)
+  sits.
+
+- When the current frame contains no `caret:` declarations:
+  - If the previous frame contained one, the rendered `output` MUST leave the
+    hardware cursor hidden.
+  - Otherwise, the rendered `output` MUST NOT include cursor-positioning or
+    cursor-visibility bytes; the caller's cursor state is preserved.
+
+The byte-level path the renderer takes to satisfy these outcomes is
+implementation-defined. The renderer MAY hide the cursor before cell writes to
+prevent flicker, MAY omit such a hide for in-place edits where flicker is
+acceptable, MAY emit only a CUP when the caret has moved within an
+already-visible state, MAY use whatever escape-sequence path it judges
+appropriate. Only the post-frame cursor state above is normative.
+
+If more than one `caret:` declaration is present in a frame, the renderer SHOULD
+use the first in directive order for the hardware cursor. Behavior for
+additional declarations is intentionally unspecified, leaving room for a future
+multi-cursor extension without breaking this contract.
+
+This responsibility is limited to the hardware cursor's position and visibility.
+Cursor shape and blink rate remain caller-managed.
+
 ---
 
 ## 8. Public Rendering API
@@ -604,7 +673,6 @@ The renderer MUST NOT emit escape sequences for any of the following
 terminal-management operations:
 
 - Entering or leaving the alternate screen buffer
-- Hiding or showing the cursor
 - Setting the cursor shape or blink state
 - Enabling or disabling mouse reporting
 - Enabling or disabling keyboard protocol modes (e.g., Kitty progressive
@@ -613,7 +681,9 @@ terminal-management operations:
 
 These are the caller's responsibility. The renderer's output contains only the
 escape sequences needed to render the frame content (cursor positioning for cell
-writes, SGR attributes for styling, and UTF-8 text).
+writes, SGR attributes for styling, and UTF-8 text) and, when a `caret`
+declaration is present, the cursor-positioning and cursor-visibility sequences
+specified in §7.6.
 
 ### 11.3 The renderer does not own application lifecycle