Improve position restoration after changing the font size

[dsilva-adastra]

Describe the bug

When changing text size Readium doesn't keep the position and a user lose the page that I was before causing them to go back or forth to find it again.

How to reproduce?

• Open an ePub
• Change the text size (e.g. on page 5) to 200%
• Text change and stays on page 5
• Navigate to next page or two pages (page 6 or 7)
• Change text size back to 100%
• It stays in the page 6 or 7 and I lose where I was before

Readium version

3.4.0

OS version

iOS 17 to 26

Testing device

iPhone 12 mini

Environment

macOS: 26.0
platform: arm64
Xcode 26.0
Build version 17A324

Additional context

I did some research that says that it should work out of the box for epubs, but I couldn't find any that works. Is that statement true?

Thank you

[mickael-menu]

This works as expected due to percent-based positioning, which is a common issue in reflowable reading systems.

However, we can still improve this. We could experiment with using text snippets to restore the currently visible text.