fix: collapse orphaned wide-char lead on trailing-column overlay

src/clayterm.c

@@ -176,6 +176,21 @@ static void present_cups(struct Clayterm *ct, int row) {
       if (w < 1)
         w = 1;
 
+      /* If a narrow glyph was floated onto a trailing column of this wide
+       * char, render_text left the lead cell but the trailing cell now
+       * holds a non-space glyph. Collapse the orphaned lead to a space and
+       * treat it as width-1 so the overlay glyph emits on the next pass. */
+      if (w > 1) {
+        for (int i = 1; i < w && x + i < ct->w; i++) {
+          Cell *bw = cell_at(ct, ct->back, x + i, y);
+          if (bw->ch != ' ') {
+            back->ch = ' ';
+            w = 1;
+            break;
+          }
+        }
+      }
+
       if (cell_cmp(back, front)) {
         /* copy to front */
         *front = *back;
@@ -222,6 +237,21 @@ static void present_lines(struct Clayterm *ct) {
       if (w < 1)
         w = 1;
 
+      /* If a narrow glyph was floated onto a trailing column of this wide
+       * char, render_text left the lead cell but the trailing cell now
+       * holds a non-space glyph. Collapse the orphaned lead to a space and
+       * treat it as width-1 so the overlay glyph emits on the next pass. */
+      if (w > 1) {
+        for (int i = 1; i < w && x + i < ct->w; i++) {
+          Cell *bw = cell_at(ct, ct->back, x + i, y);
+          if (bw->ch != ' ') {
+            back->ch = ' ';
+            w = 1;
+            break;
+          }
+        }
+      }
+
       *front = *back;
 
       emit_attr(ct, back->fg, back->bg);

test/width.test.ts

@@ -0,0 +1,54 @@
+import { beforeEach, describe, expect, it } from "./suite.ts";
+import { createTerm, type Term } from "../term.ts";
+import { close, fixed, grow, open, text } from "../ops.ts";
+import { print } from "./print.ts";
+
+const decode = (b: Uint8Array) => new TextDecoder().decode(b);
+
+describe("wide characters", () => {
+  let term: Term;
+  beforeEach(async () => {
+    term = await createTerm({ width: 12, height: 1 });
+  });
+
+  it("overlay on a wide char's trailing column clears the orphaned lead to a space", () => {
+    // X is floated onto col 1, the trailing column of 你 (cols 0-1).
+    let out = print(
+      decode(
+        term.render([
+          open("root", { layout: { width: grow(), height: grow() } }),
+          text("你好"),
+          open("ov", {
+            floating: { x: 1, y: 0, attachTo: 3 },
+            layout: { width: fixed(1), height: fixed(1) },
+          }),
+          text("X"),
+          close(),
+          close(),
+        ]).output,
+      ),
+      12,
+      1,
+    );
+    // half-overwritten 你 collapses to a space, X lands at col 1, 好 untouched at 2-3.
+    expect(out).toBe(" X好         ");
+  });
+
+  it("emits an explicit byte for an overlay landing on a placeholder column", () => {
+    let ansi = decode(
+      term.render([
+        open("root", { layout: { width: grow(), height: grow() } }),
+        text("你好"),
+        open("ov", {
+          floating: { x: 1, y: 0, attachTo: 3 },
+          layout: { width: fixed(1), height: fixed(1) },
+        }),
+        text("X"),
+        close(),
+        close(),
+      ]).output,
+    );
+    // the overlay glyph reaches the byte stream rather than being swallowed by the column skip.
+    expect(ansi).toContain("X");
+  });
+});