[css-view-transitions-2] Allow to control whether make a particular group when the named element was there only in one side #8528
Description
Can I create a named group only when elements with the same name defined by the view-transition-name property, exist in both the before and after transition?
I am trying to figure out how to include an element with marked as view-transition-name: xxx, that exists only in one side of transition, in ::view-transition-group(root) instead of ::view-transition-group(xxx).
I'm developing the tool to create HTML-based presentation slides called Marp, that have slide transitions support powered by View Transitions API. All of transition effects for slides are defined as CSS animations working in ::view-transition-group(root), and the slide author can make a morphing animation like PowerPoint Morph and Keynote Magic Move, by marking morphable elements between 2 slides by view-transition-name CSS property.
In this case, even if there is an element named with view-transition-name: xxx on only one side of the transition, a corresponding ::view-transition-group(xxx) with a simple fade effect is generated. However, this element is rendered in the same position before and after the transition due to the specification of View Transitions API. If the ::view-transition-group(root) has a flashy effect, may feel this element like out of place.
The following slide example sets various transitions when navigating, and view-transition-name property to the inline code in page 2 and 3. (Source)
example.mov
If there is no view-transition-name property on one side of the transition, I want to make that element belong to ::view-transition-group(root). (In above example, transitions when navigating 1 ↔︎ 2 and 3 ↔︎ 4)
I tried to use JavaScript to generate ::view-transition-group() only when the same named view-transition-name property exists on both sides of the transition, but it didn't work out in the end.
I also thought using ::view-transition-old(*):only-child and ::view-transition-new(*):only-child CSS selectors as already explained in https://drafts.csswg.org/css-view-transitions-1/#example-387929ce. However, in situations where this selector can be used, ::view-transition-group() has already been generated. It is not possible to change that group to belong to ::view-transition-group(root) by CSS.
I apologize if this already is achievable. But if not, are there any insights about controlling whether make a particular group when the element named by view-transition-name property was there only in one side of the transition? (e.g. Adding an optional argument to document.startViewTransition to set constraints about generating the view transition group)