Vertical Japanese text scales down instead of reflowing when decreasing font size

[Soul-Kim]

Describe the bug

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.

How to reproduce?

1、Open a vertical-rl EPUB (Japanese) in the TestApp.

2、Go to User Settings / Appearance.

3、Decrease the font size.

4、Observe that the text area shrinks toward the center instead of filling the vertical viewport height.

Readium version

3.8.0

OS version

iOS 26.4

Testing device

iPhone 17 Pro (emulator)

Environment

xcode:26.4.1 (17E202)

Additional context

Original Discussion: #790

[mickael-menu]

Thank you for reporting this issue. It's important to log it, but unfortunately, we likely won't have time to fix it anytime soon.