browser-canvas-testing

Browser Canvas Testing

Domain knowledge for using VS Code native browser tools against HTML canvas applications verified via VS Code source code review.

When to Use

• Clicking game objects or UI elements rendered inside an HTML <canvas>
• Verifying canvas visual state with screenshotPage
• Discovering why readPage returns empty on a canvas-only page
• Any Phaser 3 / WebGL / 2D canvas game interaction in browser tools

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1. Why clickElement Cannot Target Canvas Objects

clickElement is selector-based only. The VS Code source (clickBrowserTool.ts) defines these parameters:

pageId, selector, ref, dblClick, button

There is no coordinate or position parameter. The implementation does exactly:

page.locator(selector).click();

A <canvas> element renders its content via GPU/CPU drawing calls — there are no child DOM elements for selectors to target. Even canvas as a CSS selector only clicks the canvas border box center, not a specific game object inside it.

Conclusion: clickElement cannot be used for canvas game interaction. Use runPlaywrightCode instead.

Source: clickBrowserTool.ts

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2. Canvas Coordinate Conversion Formula

Canvas games use an internal game-space coordinate system that does not map directly to CSS page coordinates. Use getBoundingClientRect to convert.

// 1. Query canvas bounding rect (accounts for scaling and letterboxing)
const rect = await page.evaluate(() => {
  const c = document.querySelector("canvas");
  if (!c) throw new Error("Canvas element not found");
  const r = c.getBoundingClientRect();
  return { left: r.left, top: r.top, width: r.width, height: r.height };
});

// 2. Convert game-space coords → CSS page coords
const cssX = rect.left + (gameX / GAME_WIDTH) * rect.width;
const cssY = rect.top + (gameY / GAME_HEIGHT) * rect.height;

// 3. Click via Playwright mouse API
await page.mouse.click(cssX, cssY);

If the page has multiple <canvas> elements (e.g., a main scene canvas plus a HUD overlay), document.querySelector('canvas') returns the first one, which may not be the target. Use a specific selector instead: document.querySelector('#game canvas') or document.getElementById('game-canvas'). Inspect with page.evaluate(() => document.querySelectorAll('canvas').length) to detect multiple canvases.

Prerequisite: The canvas must be rendered before calling getBoundingClientRect(). An unrendered canvas returns {0,0,0,0}, causing the formula to click (0,0) silently. See Section 3 for the render-ready wait pattern.

Variables:

• gameX, gameY — coordinates in the game's internal resolution (e.g., from game design docs or layout)
• GAME_WIDTH, GAME_HEIGHT — the game's declared resolution (e.g., 1920, 1080 for a 1920×1080 game)
• cssX, cssY — CSS page coordinates passed to Playwright

This formula is verified working for Phaser 3 in FIT scale mode at 1920×1080.

Note — scale mode matters: In FIT mode the canvas CSS box scales to the viewport while GAME_WIDTH/GAME_HEIGHT stay constant, so this formula is exact. In RESIZE mode the game's internal resolution changes to match the viewport; use canvas.width and canvas.height read via page.evaluate() for GAME_WIDTH/GAME_HEIGHT instead of hardcoded constants. Other modes (NONE, ENVELOP) may require similar adjustments.

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3. runPlaywrightCode Usage Pattern

runPlaywrightCode executes arbitrary Playwright code against the current browser page — the correct tool for all canvas interaction.

Full worked example — click a game object at game-space position (960, 540) in a 1920×1080 game:

// Wait for canvas to be rendered and sized
await page.waitForFunction(() => {
  const c = document.querySelector("canvas");
  return c !== null && c.getBoundingClientRect().width > 0;
});

const rect = await page.evaluate(() => {
  const c = document.querySelector("canvas");
  const r = c.getBoundingClientRect();
  return { left: r.left, top: r.top, width: r.width, height: r.height };
});

const GAME_WIDTH = 1920;
const GAME_HEIGHT = 1080;
const gameX = 960;
const gameY = 540;

const cssX = rect.left + (gameX / GAME_WIDTH) * rect.width;
const cssY = rect.top + (gameY / GAME_HEIGHT) * rect.height;

await page.mouse.click(cssX, cssY);

Also useful via runPlaywrightCode:

• await page.mouse.move(cssX, cssY) — hover without clicking
• await page.keyboard.press('Space') — send key events to the game
• await page.evaluate(() => window.gameState) — read exposed JS globals for state assertions

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4. screenshotPage — Works for Canvas Verification

screenshotPage does work on canvas content. It uses VS Code's internal browserViewModel.captureScreenshot() — not Playwright's .screenshot() method. This internal capture reads the rendered frame buffer directly, so canvas pixels are captured correctly.

Use screenshotPage to:

• Verify game state visually after an interaction
• Confirm UI transitions or animations completed
• Capture evidence of pass/fail for test reports

Screenshot output: write captured images to .tmp/issue-{N}-{name}.png per skills/terminal-hygiene/SKILL.md ## Scratch & Temp-File Hygiene — never pass a C:\... absolute path to a Bash redirect.

Source: screenshotBrowserTool.ts

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5. readPage — Returns Empty for Canvas-Only Pages

readPage calls playwrightService.getSummary() which returns a DOM accessibility/semantic snapshot. Canvas elements render no accessible child nodes — the entire visual scene is opaque to the accessibility tree.

Result: readPage returns an empty or near-empty response for canvas-only applications.

Do not use readPage to:

• Detect game objects or UI state in a canvas game
• Assert text labels rendered inside the canvas
• Navigate or introspect canvas content

Use runPlaywrightCode with page.evaluate() to read game state from exposed JS APIs, or use screenshotPage for visual verification.

Source: readBrowserTool.ts

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VS Code Source Citations

All behaviors above are verified from VS Code 1.110 source code:

Tool	Source File
click_element / clickElement	clickBrowserTool.ts
screenshot_page / screenshotPage	screenshotBrowserTool.ts
read_page / readPage	readBrowserTool.ts
run_playwright_code / runPlaywrightCode	VS Code native Playwright code execution (no single tool source file — functionality available when workbench.browser.enableChatTools: true)

Gotchas

Trigger	Gotcha	Fix
Using clickElement to interact with canvas game objects	clickElement is selector-based only — no coordinate parameter exists; canvas internals are not addressable via selector	Use runPlaywrightCode with page.mouse.click(cssX, cssY) and the canvas coordinate formula
Querying getBoundingClientRect() before the canvas has rendered	Returns {0, 0, 0, 0} silently; click lands at top-left corner of page	Add await page.waitForFunction(() => document.querySelector('canvas')?.width > 0) before querying the rect
Hardcoding GAME_WIDTH/GAME_HEIGHT in RESIZE scale mode	FIT formula doesn't apply; clicks land at wrong position	Read canvas.width/canvas.height via page.evaluate() rather than using known design dimensions
Using document.querySelector('canvas') when multiple canvases exist	Always returns the first canvas — may be HUD overlay, not game scene	Use a specific selector (#game canvas) or verify canvas count first
Using readPage on canvas-only pages	Returns empty — canvas pixels are not DOM text	Use screenshotPage for visual state verification instead