Continuous scrolling is very slow for some epub books

[demeralde]

Describe the bug
Continuous scrolling is painfully slow for some epub books. I haven't noticed this in all books, it seems to depend on the size of the book.

To Reproduce
Steps to reproduce the behavior:

1. Download and open The Mind Illuminated by Culadasa epub book (I can't provide a source for this unfortunately)
2. Turn on continuous scrolling
3. Scroll down the page, it will be very choppy and slow
4. Try clicking on a section on the sidebar, it will take a very long time to load and jump to it

Expected behavior
Continuous scrolling should always be fluid and snappy, regardless of the size of the book.

Screenshots
This doesn't look or feel as choppy as it actually is because it's a gif:

Version:

• Foliate version: 2.4.2
• OS/Distribution and version: Ubuntu 20.04 LTS
• Desktop environment: Gnome 3.36.3
• Installation method: apt

[archisman-panigrahi]

It looks like continuous scrolling consumes a lot of CPU cycles.

[johnfactotum]

This is really an upstream Epub.js issue.

[demeralde]

Update: I've noticed this same issue with other books. It's also happening with the "scrolled" setting. This is a pretty big issue for me as a user, it inhibits the reading experience.

[Dreomite]

It was a problem half a year ago as well, when I started using Foliate.

The only related issue in epub.js repo I could find is futurepress/epub.js#714 which was closed 2 years ago. Should we ask to reopen it?

[demeralde]

@Dreomite I'd say so. I'll make a comment.

[demeralde]

@johnfactotum as seen in this thread: futurepress/epub.js#714

It looks like they're not going to fix this anytime soon, so we can't rely on them. However, there's a quick hotfix which can be used (mentioned in that thread). If the repo is forked, this can be used in the meantime until they fix it.

[johnfactotum]

If you're interested in working on this issue, Foliate currently uses Epub.js built from this fork: https://github.com/johnfactotum/epub.js. The workflow for now is basically running npm run build and manually copying dist/epub.js to Foliate's src/web/epub.js.

[demeralde]

@johnfactotum thanks, I'll hopefully have some time to do that. If not, if anyone else is interested feel free to go ahead because I may be a while.

[johnfactotum]

I think this can be closed now. In scrolled mode, the new renderer only calculates current location after debouncing (see #861 (comment)), which fixes the problem.

The new renderer doesn't support continuous scrolling yet, but that's a separate issue.