diff --git a/virtualizer/test/exactness.test.ts b/virtualizer/test/exactness.test.ts
new file mode 100644
index 0000000..56877ee
--- /dev/null
+++ b/virtualizer/test/exactness.test.ts
@@ -0,0 +1,81 @@
+import { describe, expect, it } from "./suite.ts";
+import { Virtualizer } from "../mod.ts";
+
+function charMeasure(text: string): number {
+  let cp = text.codePointAt(0)!;
+  if (cp >= 0x4e00 && cp <= 0x9fff) return 2;
+  if (cp < 0x20) return 0;
+  return 1;
+}
+
+describe("Exactness vs approximation structural tests", () => {
+  it("Exact viewport assertions — O-1 through O-9 hold with varied content", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 15, rows: 10 });
+    v.appendLine("short");
+    v.appendLine("a".repeat(30));
+    v.appendLine("文字abc文字def");
+    v.appendLine("\x1b[31mcolored\x1b[0m text");
+    v.appendLine("");
+
+    let vp = v.resolveViewport();
+
+    // O-1: entries ordered
+    for (let i = 1; i < vp.entries.length; i++) {
+      expect(vp.entries[i].lineIndex).toBeGreaterThan(vp.entries[i - 1].lineIndex);
+    }
+
+    for (let entry of vp.entries) {
+      // O-3: wrapPoints monotonic
+      for (let j = 1; j < entry.wrapPoints.length; j++) {
+        expect(entry.wrapPoints[j]).toBeGreaterThan(entry.wrapPoints[j - 1]);
+      }
+      // O-6: totalSubRows
+      expect(entry.totalSubRows).toBe(entry.wrapPoints.length + 1);
+      // O-7: subrow bounds
+      expect(entry.firstSubRow).toBeGreaterThanOrEqual(0);
+      expect(entry.visibleSubRows).toBeGreaterThanOrEqual(0);
+      expect(entry.firstSubRow + entry.visibleSubRows).toBeLessThanOrEqual(entry.totalSubRows);
+    }
+
+    // O-8: total visible ≤ rows
+    let totalVisible = vp.entries.reduce((s, e) => s + e.visibleSubRows, 0);
+    expect(totalVisible).toBeLessThanOrEqual(10);
+  });
+
+  it("Structural estimation non-negative — totalEstimatedVisualRows ≥ 0 after random ops", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 20, rows: 10, maxLines: 50 });
+    for (let i = 0; i < 100; i++) {
+      v.appendLine("x".repeat(i % 40));
+    }
+    v.scrollBy(-30);
+    v.resize(15, 10);
+    expect(v.totalEstimatedVisualRows).toBeGreaterThanOrEqual(0);
+  });
+
+  it("Structural estimation zero-empty — both fields 0 on empty buffer", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 80, rows: 24 });
+    expect(v.totalEstimatedVisualRows).toBe(0);
+    expect(v.currentEstimatedVisualRow).toBe(0);
+  });
+
+  it("Structural estimation positive-nonempty — totalEstimatedVisualRows > 0 when lineCount > 0", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 80, rows: 24 });
+    v.appendLine("x");
+    expect(v.totalEstimatedVisualRows).toBeGreaterThan(0);
+  });
+
+  it("Structural currentEstimate inequality — valid range when lineCount > 0", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 20, rows: 10 });
+    for (let i = 0; i < 50; i++) v.appendLine("a".repeat(i % 30));
+    v.scrollBy(-20);
+    expect(v.currentEstimatedVisualRow).toBeGreaterThanOrEqual(0);
+    expect(v.currentEstimatedVisualRow).toBeLessThan(v.totalEstimatedVisualRows);
+  });
+
+  it("Reference-behavior exact formula — estimation matches ceil(displayWidth/columns)", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 10, rows: 24 });
+    let before = v.totalEstimatedVisualRows;
+    v.appendLine("a".repeat(15)); // displayWidth=15, ceil(15/10)=2
+    expect(v.totalEstimatedVisualRows - before).toBe(2);
+  });
+});
diff --git a/virtualizer/test/resize.test.ts b/virtualizer/test/resize.test.ts
new file mode 100644
index 0000000..0521edb
--- /dev/null
+++ b/virtualizer/test/resize.test.ts
@@ -0,0 +1,185 @@
+import { describe, expect, it } from "./suite.ts";
+import { Virtualizer } from "../mod.ts";
+import { skipAnsiSequence } from "../ansi-scanner.ts";
+
+function charMeasure(text: string): number {
+  let cp = text.codePointAt(0)!;
+  if (cp >= 0x4e00 && cp <= 0x9fff) return 2;
+  if (cp < 0x20) return 0;
+  return 1;
+}
+
+function sliceAtWrapPoints(text: string, wrapPoints: number[]): string[] {
+  if (wrapPoints.length === 0) return [text];
+  let slices: string[] = [];
+  let prev = 0;
+  for (let wp of wrapPoints) {
+    slices.push(text.slice(prev, wp));
+    prev = wp;
+  }
+  slices.push(text.slice(prev));
+  return slices;
+}
+
+function stripAnsi(text: string): string {
+  let result = "";
+  let i = 0;
+  while (i < text.length) {
+    let skip = skipAnsiSequence(text, i);
+    if (skip > 0) { i += skip; continue; }
+    result += text[i];
+    i++;
+  }
+  return result;
+}
+
+function visibleWidth(text: string): number {
+  let stripped = stripAnsi(text);
+  let w = 0;
+  for (let i = 0; i < stripped.length; i++) {
+    let cp = stripped.codePointAt(i)!;
+    if (cp > 0xffff) { i++; w += 2; }
+    else if (cp >= 0x4e00 && cp <= 0x9fff) w += 2;
+    else if (cp < 0x20) w += 0;
+    else w += 1;
+  }
+  return w;
+}
+
+describe("C.RESIZE — resize", () => {
+  it("C.RESIZE.wrap-cache-invalidated — wrap points recomputed after column change", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 80, rows: 24 });
+    v.appendLine("a".repeat(100));
+    let vp1 = v.resolveViewport();
+    expect(vp1.entries[0].totalSubRows).toBe(2); // ceil(100/80)=2
+
+    v.resize(50, 24);
+    let vp2 = v.resolveViewport();
+    expect(vp2.entries[0].totalSubRows).toBe(2); // ceil(100/50)=2, but with exact wrapping
+    // Verify wrap points are different from width-80
+    expect(vp2.entries[0].wrapPoints).not.toEqual(vp1.entries[0].wrapPoints);
+  });
+
+  it("C.RESIZE.anchor-subrow-clamped — anchorSubRow clamped to new wrap count", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 20, rows: 24 });
+    v.appendLine("a".repeat(100)); // 5 sub-rows at width 20
+    v.scrollBy(4); // anchorSubRow=4
+    expect(v.anchorSubRow).toBe(4);
+
+    v.resize(50, 24); // now 2 sub-rows → clamp to 1
+    expect(v.anchorSubRow).toBe(1);
+  });
+
+  it("C.RESIZE.bottom-follow-preserved — isAtBottom survives resize", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 80, rows: 24 });
+    v.appendLine("hello");
+    expect(v.isAtBottom).toBe(true);
+    v.resize(40, 24);
+    expect(v.isAtBottom).toBe(true);
+  });
+
+  it("C.RESIZE.row-only-no-invalidation — row-only change does not affect wrapping", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 80, rows: 24 });
+    v.appendLine("a".repeat(100));
+    let vp1 = v.resolveViewport();
+    let total1 = v.totalEstimatedVisualRows;
+    let anchor1 = v.anchorLineIndex;
+
+    v.resize(80, 40);
+    expect(v.totalEstimatedVisualRows).toBe(total1);
+    expect(v.anchorLineIndex).toBe(anchor1);
+  });
+
+  it("C.RESIZE.viewport-correct-after-resize — all output invariants hold after resize", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 80, rows: 24 });
+    v.appendLine("short");
+    v.appendLine("a".repeat(100));
+    v.appendLine("文字".repeat(20));
+
+    v.resize(40, 24);
+    let vp = v.resolveViewport();
+
+    // O-1: entries ordered
+    for (let i = 1; i < vp.entries.length; i++) {
+      expect(vp.entries[i].lineIndex).toBeGreaterThan(vp.entries[i - 1].lineIndex);
+    }
+
+    for (let entry of vp.entries) {
+      // O-6: totalSubRows
+      expect(entry.totalSubRows).toBe(entry.wrapPoints.length + 1);
+      // O-7: subrow bounds
+      expect(entry.firstSubRow + entry.visibleSubRows).toBeLessThanOrEqual(entry.totalSubRows);
+      // O-9: sliced width within columns
+      let slices = sliceAtWrapPoints(entry.text, entry.wrapPoints);
+      for (let slice of slices) {
+        expect(visibleWidth(slice)).toBeLessThanOrEqual(40);
+      }
+    }
+
+    // O-8: visible rows within budget
+    let totalVisible = vp.entries.reduce((s, e) => s + e.visibleSubRows, 0);
+    expect(totalVisible).toBeLessThanOrEqual(24);
+  });
+
+  it("C.RESIZE.estimation-fields-valid-after-resize — estimation constraints hold", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 80, rows: 24 });
+    for (let i = 0; i < 100; i++) v.appendLine(`line ${i}`);
+
+    v.resize(40, 24);
+    expect(v.totalEstimatedVisualRows).toBeGreaterThan(0);
+    expect(v.currentEstimatedVisualRow).toBeGreaterThanOrEqual(0);
+    expect(v.currentEstimatedVisualRow).toBeLessThan(v.totalEstimatedVisualRows);
+  });
+
+  it("C.RESIZE.displayWidth-unchanged — cached displayWidth unchanged across resize", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 80, rows: 24 });
+    let idx = v.appendLine("hello");
+    let before = v.getLineDisplayWidth(idx);
+    v.resize(40, 24);
+    expect(v.getLineDisplayWidth(idx)).toBe(before);
+  });
+
+  it("C.RESIZE.empty-buffer — resize on empty buffer does not error", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 80, rows: 24 });
+    v.resize(40, 24);
+    let vp = v.resolveViewport();
+    expect(vp.entries.length).toBe(0);
+    expect(vp.totalEstimatedVisualRows).toBe(0);
+  });
+});
+
+describe("G.RESIZE — resize golden fixtures", () => {
+  it("G.RESIZE.narrow-to-wide — wrapping removed when columns increase", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 40, rows: 24 });
+    v.appendLine("a".repeat(80));
+    let vp1 = v.resolveViewport();
+    expect(vp1.entries[0].totalSubRows).toBe(2);
+
+    v.resize(80, 24);
+    let vp2 = v.resolveViewport();
+    expect(vp2.entries[0].totalSubRows).toBe(1);
+    expect(vp2.entries[0].wrapPoints).toEqual([]);
+  });
+
+  it("G.RESIZE.wide-to-narrow — wrapping added when columns decrease", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 80, rows: 24 });
+    v.appendLine("a".repeat(80));
+    let vp1 = v.resolveViewport();
+    expect(vp1.entries[0].totalSubRows).toBe(1);
+
+    v.resize(20, 24);
+    let vp2 = v.resolveViewport();
+    expect(vp2.entries[0].totalSubRows).toBe(4);
+    expect(vp2.entries[0].wrapPoints.length).toBe(3);
+  });
+});
+
+describe("C.APPEND.caches-displayWidth (deferred from PR 2)", () => {
+  it("C.APPEND.caches-displayWidth — displayWidth unchanged after resize", () => {
+    let v = new Virtualizer({ measureWidth: charMeasure, columns: 80, rows: 24 });
+    let idx = v.appendLine("abc");
+    expect(v.getLineDisplayWidth(idx)).toBe(3);
+    v.resize(2, 24);
+    expect(v.getLineDisplayWidth(idx)).toBe(3);
+  });
+});
diff --git a/virtualizer/virtualizer.ts b/virtualizer/virtualizer.ts
index 22fd18e..ad9c258 100644
--- a/virtualizer/virtualizer.ts
+++ b/virtualizer/virtualizer.ts
@@ -135,8 +135,46 @@ export class Virtualizer {
     return newLineIndex;
   }
 
-  resize(_columns: number, _rows: number): void {
-    throw new Error("not implemented");
+  /**
+   * resize(columns, rows) — §10.3
+   *
+   * On column-width change: clear wrap cache, recompute estimation,
+   * clamp anchor sub-row, recompute currentEstimatedVisualRow.
+   * On row-only change: just update rows.
+   */
+  resize(columns: number, rows: number): void {
+    if (columns !== this._columns) {
+      // Column width changed — invalidate wrap cache (INV-5)
+      this._wrapCache.clear();
+
+      // Recompute totalEstimatedVisualRows with new column width
+      let newTotal = 0;
+      let baseIndex = this._ringBuffer.baseIndex;
+      for (let i = baseIndex; i < baseIndex + this._ringBuffer.lineCount; i++) {
+        let entry = this._ringBuffer.get(i);
+        if (!entry) break;
+        newTotal += Math.max(1, Math.ceil(entry.displayWidth / columns));
+      }
+      this._totalEstimatedVisualRows = newTotal;
+
+      // Clamp anchor sub-row at new width
+      if (this._ringBuffer.lineCount > 0) {
+        let anchorEntry = this._ringBuffer.get(this._anchorLineIndex);
+        if (anchorEntry) {
+          let newEstimate = Math.max(1, Math.ceil(anchorEntry.displayWidth / columns));
+          if (this._anchorSubRow >= newEstimate) {
+            this._anchorSubRow = newEstimate - 1;
+          }
+        }
+      }
+
+      this._columns = columns;
+
+      // Recompute currentEstimatedVisualRow
+      this._recomputeCurrentEstimate();
+    }
+
+    this._rows = rows;
   }
 
   /**