TextLayer not well aligned for Amharic fonts extracted from this PDF

[technicaltitch]

File:
Social Studies in Amharic Grade 5 Student Book.pdf

Configuration:

• Web browser and its version:Chrome 79
• Operating system and its version:macOS 10.14.6
• PDF.js version:9a437a1
• Is a browser extension:No

Steps to reproduce the problem:

1. Load the attached file
2. Select non-justified lines by dragging down the left margin of the PDF, and the selection is not aligned with the text displayed on the canvas. (Similar happens when double-clicking to select a word, etc.) This is because the font is loaded from the PDF for the canvas, but not used on the TextLayer, which contains strings such as " ôÇ=^L© ÈV¡^c=Á© ]ùwK=¡ ¾ƒUIƒ T>’>e‚ " (the right character codes in a standard font).

What is the expected behavior? (add screenshot)
The selected area should cover the text displayed. I presume this means loading the fonts from the PDF into the TextLayer, and that this is not done to improve performance (ref), so would a setting to turn off this optimization be possible? (I tried the textLayerMode or enhanceTextSelection settings, and adding an extracted font-family codename to the TextLayer span CSS.)

I have generated a screenshot of expected behaviour by rendering using SVG, and deleting the TextLayer div:

Going by the deeper shading, I suspect my diagnosis is nonsense and there are several spans overlaid. (Unfortunately I have textbooks for the entire school curriculum, and I'm trying to launch for lock down study, I only have evenings, so am stuck with these awful PDFs. )

I would love to extract the font though so I can extract and display text outside pdf.js, if possible. Any pointers very gratefully received - I'm still struggling to navigate the core.

What went wrong? (add screenshot)
As you can see from the first screenshot, the selected areas do not match the text areas:

In the second screenshot you can faintly see the actual characters on the TextLayer spans (I made the span color black):

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):
I am running the latest code from Git in Node.

[Snuffleupagus]

Fixed by PR #13424 (at least the alignment part since copying only produces "garbage", however that can be reproduced in Adobe Reader as well).