Couple of notes regarding the updated mockup:
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On your first screenshot, let’s say I want to move event #3 to nest it under #1 instead of #2. I would expect its siblings (#4-8) to follow it, because they are attached to it. But only #3 would be moved. #4 would become attached to #2 and I guess the whole numbering would change? This doesn’t seem intuitive to me.
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On “both groups collapsed”, you nested the subgroup when I think it shouldn’t be.
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The last screenshots are very confusing because in compact mode, you’re using the normal tree structure, and when expanded, you’re switching to your inverted mode (as explained below). And as before, I think #8 should be attached to its parent instead of being attached to its sibling.
Back to the theory now…
I agree that too much space is often wasted horizontally on widescreens, even though sometimes, that empty space is welcome:
Currently, children (nestings) are shown horizontally and event sequence is shown vertically.
What you’re suggesting is inverting that logic: the event sequence becomes horizontal and the nesting becomes vertical. But the event sequence can be very long (much longer than a screen width) while the nesting is usually limited to, say, around five levels, from my personal experience so far.
What would happen then is that we would need horizontal scrolling to display the event sequence, which goes against the default wheel behavior.
To me, the solution to this widescreen issue is to do what some of the other engines do: display the scene on one side, and the events next to it (and I would leave the current double tab system for small screens).