`FormattedFrame` in Kotlin Notebook does not render HTML

[Jolanrensen]

Using df.format {}.with {} in Kotlin Notebook currently displays an ordinary native table.
These do not support formatting at the moment.

This is unexpected behavior, as a user would exect a formatted DataFrame.

Not only is this unexpected, but it's tediously difficult to get the FormattedFrame rendered according to the way
the user defined it:

• FormattedFrame.toStandaloneHtml() completely ignores any user-formatting when output in notebooks.
• FormattedFrame.toHtml() does work for changing the background color of cells, but text color is ignored (textColor does not work for number colums #251)
• FormattedFrame.toHtml().toString().let(::HTML) seems to be the only way to print the DataFrame exactly how it's defined

We should probably make it so that a FormattedFrame gets rendered as HTML by default until native DataFrame tables in notebooks support formatting themselves.

[Jolanrensen]

After discussion, we likely want to encode whether a DataFrame is a FormattedFrame in the embedded JSON we send to Kotlin Notebook. Then they can make the decision to show either a native DataFrame or HTML.

This keeps makes it easier in the future to allow the format DSL to apply inside native tables as well. (NOTE that this requires an overhaul of said DSL, as it produces straight HTML+CSS atm and allows for custom attributes that would need to be translated to Swing)

Relevant: https://jetbrains.team/p/ij/repositories/ultimate/files/11566b056b3d30a3fc8a3f50ed2892ee957d600d/plugins/kotlin/jupyter/tables/src/com/intellij/kotlin/jupyter/tables/KotlinDataframeOutputDataKeyExtractor.kt?tab=source&line=59&lines-count=1

[Jolanrensen]

Interestingly, toStandaloneHtml() triggers the GitHub fallback notebooks lacking any formatting too