From fcde78abad77db1e0cf95f32bb242feaa01bdb70 Mon Sep 17 00:00:00 2001
From: Ilia Mashkov <hikkikomori7@gmail.com>
Date: Sat, 11 Apr 2026 15:48:47 +0300
Subject: [PATCH] test: add canvas mock helper for pretext-based engine tests

---
 src/shared/lib/helpers/__mocks__/canvas.ts | 29 ++++++++++++++++++++++
 1 file changed, 29 insertions(+)
 create mode 100644 src/shared/lib/helpers/__mocks__/canvas.ts

diff --git a/src/shared/lib/helpers/__mocks__/canvas.ts b/src/shared/lib/helpers/__mocks__/canvas.ts
new file mode 100644
index 0000000..326de30
--- /dev/null
+++ b/src/shared/lib/helpers/__mocks__/canvas.ts
@@ -0,0 +1,29 @@
+// src/shared/lib/helpers/__mocks__/canvas.ts
+//
+// Call installCanvasMock(fn) before any pretext import to control measureText.
+// The factory receives the current ctx.font string and the text to measure.
+import { vi } from 'vitest';
+
+export type MeasureFactory = (font: string, text: string) => number;
+
+export function installCanvasMock(factory: MeasureFactory): void {
+    let currentFont = '';
+
+    const mockCtx = {
+        get font() {
+            return currentFont;
+        },
+        set font(f: string) {
+            currentFont = f;
+        },
+        measureText: vi.fn((text: string) => ({ width: factory(currentFont, text) })),
+    };
+
+    // HTMLCanvasElement.prototype.getContext is the entry point pretext uses in DOM environments.
+    // OffscreenCanvas takes priority in pretext; jsdom does not define it so DOM path is used.
+    Object.defineProperty(HTMLCanvasElement.prototype, 'getContext', {
+        configurable: true,
+        writable: true,
+        value: vi.fn(() => mockCtx),
+    });
+}