Every canvas starts with a 300 by 150 bitmap, so a nonzero width does not mean it was drawn

Fact

A <canvas> without width and height attributes has a transparent 300 by 150 bitmap from the moment it exists, and canvas.width reads 300. CSS sizing only scales that bitmap. The size changes only when code assigns it, typically in the app's first draw.

Why it matters

A test that waits for canvas.width > 0 to know the app has drawn waits for nothing. The condition is true at once, and the pixel read after it comes back transparent black whenever the first draw is late: a slow first frame under software rendering, or a frame loop that has not ticked yet. So it passes on fast machines and fails intermittently on CI.

The same assumption hides in guards such as c.width ? read(c) : null, whose null branch never runs.

How to apply

• Wait for something the draw itself sets: the size the app assigns (a square canvas is never 300 by 150, so width === height), a pixel, or a flag the app exposes.
• If "not drawn yet" should be observable, declare width="0" height="0" in the markup and let the first draw size the canvas.
• Assigning the size also clears the canvas: [[resizeobserver-first-callback-clears-the-canvas]].