After changes in e4253633c9 the transition page content wasn't properly centered anymore in the single/double reading mode depending on the applied reader settings (scale type, stretch, reader width)
Due to handling an "offset spread page" change as if the "double page" mode got activated, the index of the secondary page was set, which resulted in opening the next page
Depending on the screen size and e.g. the set "reader width" pages might not have filled out the full img elements size due to "object-fit contain".
This was caused due to the image wrappers flex container "align-items center" which slightly reduced the images width to some ".xy" number (e.g. instead of wrappers width 258px, the images were something like 257.83px)
In case two pages were displayed with different dimensions, the bigger page might take up more than 50% of the screen.
In this case the bigger page got limited to 50% which caused in to unintentionally get reduced in size
In case the last page of a manga was immediately visible or the became visible after the "image spread state" change, the chapter never got marked as read
There was an issue where one had to click twice at the end of a chapter in the "double page mode" to get to the chapter transition page.
This was caused due to setting the "primary page index" as the "current page index" and thus, when opening the next page, it got changed to the "secondary page index". However, this page was already visible and therefore, there was no visual change.
Due to the way the pages got scrolled into view, the current page was not changed to the new page.
This then prevented the previous/next page to get opened via click/hotkey actions
