Vertical EPUB rendering issue: Text area scales down instead of reflowing (tested with latest TestApp)

[Soul-Kim]

Environment:

Toolkit: readium/swift-toolkit (Latest main branch)

App tested: The official TestApp included in the repository

Device: iPhone 17 Pro (Simulator / Physical device)

Description:
I am experiencing a layout issue with vertical-writing EPUBs (Japanese). When I decrease the font size, the entire text content area shrinks (as if it's being scaled down), creating large white margins at the top and bottom.

I have confirmed that this issue is consistent with the behavior in the latest official TestApp provided in this repository. It does not seem to be a custom implementation error but rather a rendering behavior of the navigator or Readium CSS in vertical mode.

Expected behavior:
The text should reflow to fill the available height of the viewport, regardless of the font size.

Screenshots:

This message was translated by AI. Please excuse any grammatical errors or awkward phrasing as my English is limited.

[mickael-menu]

Hi, could you open an issue instead of a discussion, with the Bug Report issue template? Thank you
https://github.com/readium/swift-toolkit/issues

[Soul-Kim]

Hi, I have opened the issue here: #791

Please note that I am a beginner developer and still learning how to navigate GitHub and this framework. I appreciate your patience and help!