diff --git a/DESIGN.md b/DESIGN.md
index 94a0ac0e66..ec14f6d1da 100644
--- a/DESIGN.md
+++ b/DESIGN.md
@@ -545,6 +545,14 @@ The values are the ones in `src/styles.css`. Every token but `--fs-input` is `ca
   name `"JetBrains Mono Web"`. The name is the app's own so that the terminal never takes it:
   xterm's stack (`src/lib/fontFamily.ts`) names `"JetBrains Mono"`, sizes every cell from the
   first font that matches, and keeps meaning the copy installed on the device.
+- Except in Ovis for Mac's window: WebKit there draws a font the user installed, but its canvas,
+  which xterm and the glyph fit measure with, sees only the system's fonts. The terminal leaves
+  such a font out of its stack (`measurableFontStack`) and draws what Safari draws, Menlo and
+  Apple SD Gothic Neo, rather than a grid measured in one font and drawn in another. It follows
+  Zed and Warp, whose terminals size a cell with the font that draws it. VS Code's xterm.js
+  terminal, the nearest web one, never meets the case: in Electron the canvas and the page see
+  the same installed fonts. A WKWebView's canvas cannot, so here the font is dropped rather than
+  measured.
 - Chat reading scale: prose, the user's bubble and live narration are `--fs-chat` on
   `--lh-prose`; `strong` is `--fw-semibold` in `--text-strong`; inline code and code file chips
   are `0.87em` of their line; a code block is `--fs-sm` on `--lh-code`; headings are h1 `--fs-xl`,
diff --git a/changelog.d/mac-app-installed-font-terminal.md b/changelog.d/mac-app-installed-font-terminal.md
new file mode 100644
index 0000000000..d1dc32ab65
--- /dev/null
+++ b/changelog.d/mac-app-installed-font-terminal.md
@@ -0,0 +1,5 @@
+### Fixed
+- In Ovis for Mac's window, the terminal no longer draws Korean larger than its cells, or rows over
+  each other, on a Mac with fonts such as JetBrains Mono or D2Coding installed: the app's web view
+  draws an installed font but cannot measure it, so the terminal now uses the system's fonts there,
+  as in Safari.
diff --git a/src/lib/fontFamily.test.ts b/src/lib/fontFamily.test.ts
index bc7e32b275..9125cdfa5a 100644
--- a/src/lib/fontFamily.test.ts
+++ b/src/lib/fontFamily.test.ts
@@ -1,7 +1,7 @@
 import { afterEach, describe, expect, it } from "bun:test";
 import { readFileSync } from "node:fs";
 
-import { chatFontStack, loadFontStack, TERMINAL_FONT_STACK, terminalFontStack } from "./fontFamily.ts";
+import { chatFontStack, type FontWidths, loadFontStack, measurableFontStack, TERMINAL_FONT_STACK, terminalFontStack } from "./fontFamily.ts";
 
 describe("terminal font stack", () => {
   it("is the built-in stack when nothing is typed", () => {
@@ -34,6 +34,41 @@ describe("terminal font stack", () => {
   });
 });
 
+describe("fonts the canvas cannot measure", () => {
+  // a page that has the fonts in `drawn` and a canvas that has those in `measured`: a list is as wide
+  // as its first font either one has, and every monospaced face is as wide as monospace
+  const page = (drawn: readonly string[], measured: readonly string[]): FontWidths => {
+    const width = (has: readonly string[]) => (family: string): number => {
+      const names = family.split(",").map((name) => name.trim().replace(/["']/g, ""));
+      const first = names.find((name) => has.includes(name) || name === "monospace" || name === "serif");
+      return first === "serif" ? 280 : first === "D2Coding" ? 270 : 300;
+    };
+    return { drawn: width(drawn), measured: width(measured) };
+  };
+  const system = ["Menlo", "Monaco"];
+  const installed = ["JetBrains Mono", "D2Coding"];
+
+  it("leaves out the installed fonts Ovis for Mac's web view draws but its canvas cannot measure", () => {
+    const widths = page([...system, ...installed], system);
+    expect(measurableFontStack(TERMINAL_FONT_STACK, widths)).toBe(TERMINAL_FONT_STACK.replace('"JetBrains Mono", ', "").replace('"D2Coding", ', ""));
+    // a font typed in Settings goes the same way
+    expect(measurableFontStack(terminalFontStack("D2Coding"), widths)).toBe(TERMINAL_FONT_STACK.replace('"JetBrains Mono", ', "").replace('"D2Coding", ', ""));
+  });
+
+  it("keeps the stack where the canvas measures what the page draws, as in Chromium and Safari", () => {
+    expect(measurableFontStack(TERMINAL_FONT_STACK, page([...system, ...installed], [...system, ...installed]))).toBe(TERMINAL_FONT_STACK);
+    expect(measurableFontStack(TERMINAL_FONT_STACK, page(system, system))).toBe(TERMINAL_FONT_STACK);
+  });
+
+  it("keeps the bundled icon font, which the canvas measures once it has loaded", () => {
+    expect(measurableFontStack(TERMINAL_FONT_STACK, page(["Symbols Nerd Font Mono", ...system], system))).toBe(TERMINAL_FONT_STACK);
+  });
+
+  it("measures nothing without a page", () => {
+    expect(measurableFontStack(TERMINAL_FONT_STACK)).toBe(TERMINAL_FONT_STACK);
+  });
+});
+
 describe("chat font stack", () => {
   it("keeps the UI font when nothing is typed", () => {
     expect(chatFontStack("")).toBeNull();
diff --git a/src/lib/fontFamily.ts b/src/lib/fontFamily.ts
index abaf74091b..3011249b39 100644
--- a/src/lib/fontFamily.ts
+++ b/src/lib/fontFamily.ts
@@ -69,6 +69,78 @@ export function terminalFontStack(family: string): string {
   return chosen === "" ? TERMINAL_FONT_STACK : `${SYMBOLS_FONT}, ${chosen}, ${TERMINAL_TEXT_FONTS}`;
 }
 
+/** The width of PROBE_TEXT in a `font-family` list: as the page draws it, and as a canvas measures it. */
+export interface FontWidths {
+  drawn(family: string): number;
+  measured(family: string): number;
+}
+
+// narrow and wide Latin, a digit and Hangul: any text face changes the width of some of them
+const PROBE_TEXT = "mmmmmmmmmmlli1WW가나다";
+const PROBE_SIZE = "32px";
+// a monospaced face can match monospace's Latin to the pixel, never serif's as well
+const PROBE_FALLBACKS = ["monospace", "serif"] as const;
+const PROBE_TOLERANCE_PX = 0.5;
+const GENERIC_FAMILIES = new Set(["monospace", "serif", "sans-serif", "cursive", "fantasy", "system-ui", "ui-monospace", "ui-serif", "ui-sans-serif", "ui-rounded", "math", "emoji", "fangsong"]);
+
+let pageWidths: FontWidths | undefined;
+
+/** This page's measurements, each made once: null without a document to measure in. */
+function browserFontWidths(): FontWidths | null {
+  if (pageWidths) return pageWidths;
+  const context = typeof document === "undefined" ? null : document.createElement("canvas").getContext("2d");
+  if (!context || !document.body) return null;
+  const widths = new Map<string, number>();
+  const once = (kind: string, measure: (family: string) => number) => (family: string): number => {
+    const key = `${kind} ${family}`;
+    let width = widths.get(key);
+    if (width === undefined) {
+      width = measure(family);
+      widths.set(key, width);
+    }
+    return width;
+  };
+  pageWidths = {
+    drawn: once("drawn", (family) => {
+      const span = document.createElement("span");
+      span.style.cssText = "position:absolute;visibility:hidden;white-space:pre;letter-spacing:0";
+      span.style.fontSize = PROBE_SIZE;
+      span.style.fontFamily = family;
+      span.textContent = PROBE_TEXT;
+      document.body.append(span);
+      const width = span.getBoundingClientRect().width;
+      span.remove();
+      return width;
+    }),
+    measured: once("measured", (family) => {
+      context.font = `${PROBE_SIZE} ${family}`;
+      return context.measureText(PROBE_TEXT).width;
+    }),
+  };
+  return pageWidths;
+}
+
+function finds(width: (family: string) => number, name: string): boolean {
+  return PROBE_FALLBACKS.some((fallback) => Math.abs(width(`${name}, ${fallback}`) - width(fallback)) > PROBE_TOLERANCE_PX);
+}
+
+/**
+ * The stack without the families this page draws but its canvas cannot measure. WebKit in an
+ * app's web view (Ovis for Mac's window) draws text in a font the user installed, while its
+ * canvas sees only the system's fonts; Safari hides an installed font from both, Chromium shows
+ * it to both. xterm sizes its cells from a canvas (TextMetrics), and lib/terminalGlyphs.ts fits
+ * wide glyphs from one: with JetBrains Mono and D2Coding installed, the app measured a grid in
+ * Menlo and drew it in those, rows too short for their glyphs, and Hangul it measured as Apple SD
+ * Gothic Neo was enlarged past its two cells. Left out, such a family falls back as if it were not
+ * installed, and the terminal draws what Safari draws. The icon font and generic families stay.
+ */
+export function measurableFontStack(stack: string, widths: FontWidths | null = browserFontWidths()): string {
+  if (!widths) return stack;
+  const names = stack.split(",").map((name) => name.trim());
+  const kept = names.filter((name) => name === SYMBOLS_FONT || GENERIC_FAMILIES.has(name) || !finds(widths.drawn, name) || finds(widths.measured, name));
+  return kept.length === names.length ? stack : kept.join(", ");
+}
+
 /** The chat body's `font-family`, or null to keep the UI font. */
 export function chatFontStack(family: string): string | null {
   const chosen = sanitizeFontFamily(family);
diff --git a/src/lib/useTerminalMount.ts b/src/lib/useTerminalMount.ts
index 146f4a79d4..9a7f8344ab 100644
--- a/src/lib/useTerminalMount.ts
+++ b/src/lib/useTerminalMount.ts
@@ -16,7 +16,7 @@ import { paneStorageId } from "../../shared/machines.ts";
 import { secretPrompt } from "../../shared/secret-prompt.ts";
 import type { ClientRole, ServerMessage } from "../../shared/protocol.ts";
 import { terminalTheme, type Palette, type ResolvedTheme, type Settings } from "./settings.ts";
-import { TERMINAL_FONT_STACK } from "./fontFamily.ts";
+import { measurableFontStack, TERMINAL_FONT_STACK } from "./fontFamily.ts";
 import type { Translate } from "./i18n.ts";
 import { isAppShortcut } from "./shortcuts.ts";
 import { keyboardHoldsGrid, keyboardUp } from "./keyboard.ts";
@@ -187,7 +187,7 @@ export function useTerminalMount(deps: TerminalMountDeps): void {
         allowProposedApi: true,
         fontSize: terminalFontSize,
         // a chosen family follows in the font effect below, once its faces have loaded
-        fontFamily: TERMINAL_FONT_STACK,
+        fontFamily: measurableFontStack(TERMINAL_FONT_STACK),
         theme: terminalTheme(theme, palette),
         // a program's own colors suit one background (Claude Code's question titles are #ffffff, near
         // 1:1 on a light palette): xterm moves a foreground under 4.5:1 toward black or white. Dim text
diff --git a/src/lib/useTerminalRefits.ts b/src/lib/useTerminalRefits.ts
index 30b0dd5711..be4556abf1 100644
--- a/src/lib/useTerminalRefits.ts
+++ b/src/lib/useTerminalRefits.ts
@@ -1,6 +1,6 @@
 import { useEffect, useRef, type RefObject } from "react";
 import { terminalTheme, type Palette, type ResolvedTheme } from "./settings.ts";
-import { loadFontStack, TERMINAL_FONT_STACK, terminalFontStack } from "./fontFamily.ts";
+import { loadFontStack, measurableFontStack, TERMINAL_FONT_STACK, terminalFontStack } from "./fontFamily.ts";
 import { modalOpen } from "./focus.ts";
 import { inUse } from "./paneTerminalShared.ts";
 import type { TerminalPeek } from "./terminalPeek.ts";
@@ -60,9 +60,10 @@ export function useTerminalRefits(deps: TerminalRefitsDeps): void {
       // a newer font, or an unmounted terminal, took over while the faces loaded
       if (superseded || termRef.current !== term) return;
       baseFontRef.current = terminalFontSize;
-      if (term.options.fontSize === terminalFontSize && term.options.fontFamily === fontFamily) return;
+      const family = measurableFontStack(fontFamily);
+      if (term.options.fontSize === terminalFontSize && term.options.fontFamily === family) return;
       term.options.fontSize = terminalFontSize;
-      term.options.fontFamily = fontFamily;
+      term.options.fontFamily = family;
       if (observeRef.current || fixedGridRef.current || chatViewRef.current || keyboardHoldsRef.current(true)) return;
       try {
         fitRef.current?.fit();