Offer per-slide layout and deck-level defaults

[bollwyvl]

Elevator Pitch

A notebook-based deck should be able to be customized and then overridden on a per-cell basis.

Motivation

• CSS grid offers a lot of options for in-plane layout
• even wilder things are possible without actually changing the ordering/nesting of the DOM elements
• CSS variables can be overridden in style, including jupyterlab defaults like fine-grained font size choices, and would have very high specificity

Design Ideas

• Allow a notebook to have juptyerlab-deck.slideStyle.
• Allow slide and subslide cells to set jupyterlab-deck.slideStyle, overriding the style.
  • These would be applied to the .jp-Notebook's style, rather than the cell's.
  • These would be "sticky" until the next (sub)slide changes them, such as signaling a reversion to the default flexbox model.
• Offer a number of pre-built patterns with wee icons, which can then be updated by e.g. settings, other (theme) extensions, or copy/pasted from metadata.
  • Specifically for grid, individual cells could override the template behavior with additional metadata in jupyterlab-deck.style, though this might create illogical navigation patterns and exciting failure modes when adding too many cells for a layout, but caveat emptor once you start messing with style directly.
• Presumably Add foreground, background layers #15's position: fixed elements would not affect custom layouts.