Introduce tests for CSS-only zooming

[timvandermeij]

In PR #18077 we found that we currently don't have any tests for the CSS-only zooming functionality in the viewer. We should ideally implement two (most likely integration) tests:

• a first (basic) test that zooms in to the point where CSS-only zooming is activated and that asserts that the canvas is still rendered correctly;
• a second test that serves as a regression test for [api-minor] Respect the drawingDelay also when CSS-only zooming is used (issue 18022) #18077 (see [api-minor] Respect the drawingDelay also when CSS-only zooming is used (issue 18022) #18077 (comment)).

/cc @nicolo-ribaudo

[nicolo-ribaudo]

I opened #18129 for the second point. Regarding the first point, I tried but I cannot figure out how to get hasRestrictedScaling to be true (in the integration tests browser window) at

pdf.js/web/pdf_page_view.js

Line 655 in 9ee7c07

if (this.#hasRestrictedScaling) {

I could check how big is exactly the window on my machine and which display density it reports, but it feels very brittle and specific to my setup.

[timvandermeij]

I'm not 100% sure, but looking at

pdf.js/web/pdf_page_view.js

Lines 1014 to 1023 in 9ee7c07

	} else if (this.maxCanvasPixels > 0) {
	const pixelsInViewport = width * height;
	const maxScale = Math.sqrt(this.maxCanvasPixels / pixelsInViewport);
	if (outputScale.sx > maxScale || outputScale.sy > maxScale) {
	outputScale.sx = maxScale;
	outputScale.sy = maxScale;
	this.#hasRestrictedScaling = true;
	} else {
	this.#hasRestrictedScaling = false;
	}

I wonder if setting maxCanvasPixels to a low value might already suffice to trigger CSS-only zooming? I think it could follow the same pattern as you already introduced in #18129. I agree that we shouldn't depend on the environment the test is run in; ideally we can simply force the viewer to use CSS-only zooming after e.g. a certain zoom percentage (the current viewer should already do that; try zooming to 900% and notice that the canvas doesn't change in terms of width/height attribute anymore in the web console).