[GTK4] Navigation and rendering issues with fixed-layout documents with `page-spread-center` about foliate

[johnfactotum]

Fixed with johnfactotum/foliate-js@99e246e and johnfactotum/foliate-js@0cebcb9.

center gap between the two pages

I think this is due to the default margin on the body element, which can be fixed with body { margin: 0; }.

[initramfs]

This might be an extremely minor nitpick but even with margin: 0; padding: 0 on the html and body elements there still exists a 1 pixel gap between the spreads:

Thorium renders both variants (stylesheet-less and with stylesheet) as truly gapless:

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Relevant EPUBs here (contains original EPUB + EPUB with stylesheet): FXL-Test.zip

On another note: I'm not well versed with any specifications of default values for CSS properties besides what is shown here (I'm not sure if this is normative), but should the body margin be set to 0 px for FXL content?

[johnfactotum]

A slight gap is visible in your Thorium screenshot as well. I think it's an antialiasing artifact (antialiasing being applied to the individual pages which causes the gap to appear). As far as I know it's not really something that can be fixed as the antialiasing is controlled by the browser engine. It works for me, though, when using 200% HiDPI scaling:

Regarding the default user agent stylesheet, as far as I can tell, there aren't any special requirements for fixed layout. For EPUB in general, the reading system spec says

a reading system's user agent style sheet SHOULD support the [html] suggested default rendering.

[initramfs]

Okay, I can reproduce the gapless rendering with 200% scale as well, also noticed Thorium's gapless rendering might have a bug as well (where the pages are actually overlapped by 1 or 2 pixels, producing the AA artifacts), though that's hard to confirm (increasing zoom level causes pages to definitively overlap on my system though).

In any case, thanks for fixing this bug, seems to be the only FOSS epub renderer right now that can actually render FXL RTL correctly.