Park support of pagination for vertical writing

[JayPanoz]

I'm submitting a warning/decision.

Short description of the issue/suggestion:

It's been decided to park/pause support of pagination for vertical writing (right to left, top to bottom) as we can’t offer what implementers/consumers/users expect in CSS.

There has been several issues reported here and in mobile toolkits, and there is no way we can resolve them in the current state of CSS fragmentation (mainly CSS Multicolumn):

• Paged vertical text book should be scrollable horizontally #96
• Horizontal page turning of vertical text swift-toolkit#370
• [Bug] Chinese epub vertical documents cannot be read normally kotlin-toolkit#458

As mentioned by @danielweck, ReadiumCSS’ “pagination” is disabled in Thorium when rendering vertical writing, and re-implemented in the sense of scrolling the viewport horizontally by an offset corresponding to the visible scroll extent.

@chocolatkey also explained this is typically handled programmatically, as it can be done efficiently thanks to characters being (mostly) monospace.

Any help from specialists is welcome though. Parking doesn’t mean abandoning, we’re just trying to be pragmatic as we can’t achieve what people expect due to spec limitations at the time being.