Border colors 2

[Marc-Andre-Rivet]

This is a continuation of the previous border colors PR. This takes the essential elements from the PR and while keeping the same underlying ideas that were developed there, makes the implementation a bit tighter.

• use subset of border** CSS rules to style the table's borders (border, border-left, border-right, border-bottom, border-top) --> while only supporting a subset might feel limiting, it covers pretty much all cases with very little redundancy vs. the full rules set and simplifies implementation greatly
• omit all border** CSS rules from general cell/data/headers/filter/ops styling
• apply default styling
• reconcile common borders between various types & with n_fixed_rows & n_fixed_columns (priority order: data > filter > headers > operations > fillers)

n_fixed_rows, n_fixed_columns cases are handled through CSS: https://github.com/plotly/dash-table/pull/424/files#diff-02b1c6154ad8311a2face953de21380dR159

headers, filters, data and operations are reconciled with priority

1. data
2. filters
3. headers
4. operations

Given that all the items above share style_cell and style_cell_conditional, and that in all cases these are the lowest weight styles applied,

p1 will win on p2 if:

• p1's edge is not the table's default and not from style_cell or style_cell_conditional

• p1's edge weight is greater than p2's edge weight -- the weight is equal to the source style's index

• unit tests for edge matrices generation

• visual tests for table rendering

Note: Percy diffs will be all over the place because of the pixel-differences between box-shadow and border rendering. We'll need to be careful with that one.

[Marc-Andre-Rivet]

Just defining some types to do TypeScript stuff later..

[Marc-Andre-Rivet]

@alexcjohnson This is finally ready for another round. It's a bit of a mess by now though :(

[Marc-Andre-Rivet]

Found more issues while demoing this to @cldougl

• ops columns do not evaluate conditional styles correctly
• non-border rules do not get applied on ops cells
• op cells are misaligned in fixed sections with ☝️ applied
• filter columns with invalid values get erased when a new valid value is entered
• need to check out impact on select_cells highlight (this behaves as before)
• changelog
• need to do some code clean up
• dash docs update

[alexcjohnson]

Good grief, that was a heck of a lot bigger than it seemed it would be. I have no more comments to make, this is super thorough. 💃