Fix #9820 Thumbnail scale

[YipingRuan]

#9820

To reproduce the bug

• Set monitor DPI/Scale beyond 100%
• Thumbnails of pages out of view (not in the getVisibleElements from ui_utils.js)

[Snuffleupagus]

Why is this necessary though, since the use of getOutputScale in _getPageDrawContext should already handle things?
Can you please compare the PDFPageView and PDFThumbnailView implementations to figure out exactly where they differ? As-is, it's not entirely clear (at least to me) where the bug actually is and why the current solution is correct.

Finally, please remember to write a good commit message as well (only including information in the PR description isn't enough).

[YipingRuan]

Apparently it was not handled and the thumbnails have wrong scale at high DPI.

My guess is in pdf_page_view.js it use:

const outputScale = getOutputScale(ctx);

Not the case for "PDFThumbnailView".

In fact I copy the window.devicePixelRatio from getOutputScale. Any other hint?

I will force push to include the commit message.

[Snuffleupagus]

My guess is in pdf_page_view.js it use:

const outputScale = getOutputScale(ctx);

Not the case for "PDFThumbnailView".

Huh, it's most definitely used there as well; please see

pdf.js/web/pdf_thumbnail_view.js

Line 245 in 59b3560

const outputScale = getOutputScale(ctx);

The question is why it's not working as-is, and looking at that method in detail I've got an idea for the actual underlying bug.

[YipingRuan]

Ok great! Leave to expert then.

Other observation is "this.scale" here was always a fixed value, regardless of screen DPI.

pdf.js/web/pdf_thumbnail_view.js

Line 337 in 59b3560

const drawViewport = this.viewport.clone({ scale: this.scale });

[timvandermeij]

It looks like PR #12618 will solve the underlying problem here, so let's close this one. Thanks.