Thank you for Split Preview — it's a great improvement to the preview UX. Including the toolbar in the preview can be problematic, especially if the project uses Admin Toolbar → with Admin Toolbar Extra Tools enabled. The dropdown appears when transitioning from full screen back to the starting width. See the quick demo below.
Consider preventing toolbar rendering with hook_preprocess_html() that only executes for split_preview iframed content.
Merge request to follow.
Needs review
1.0
User interface
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Issue #3274079 by justcaldwell: Remove Admin Toolbar from preview