[Table] Fix interaction-bar styles

[cmslewis]

Changes proposed in this pull request:

• Show cursor: grab when mousing over interaction bar while reordering is enabled.
• Fix header menu's gradient background color when selection is disabled (background gets darker on hover when selection is enabled, but the background color doesn't change on hover when selection is disabled)

Reviewers should focus on:

• Does all the SCSS look okay?
• Do the styles look good when:
  • selection is enabled/disabled?
  • interaction bar is enabled/disabled?

Screenshot

[blueprint-bot]

Fix header menu styles when selection disabled

Preview: documentation | table
Coverage: core | datetime

[llorca]

Nice stuff. In addition, can we add the "drag handle" icon to the left of the interaction bar (currently missing when both the interaction bar and reordering are enabled)? It would allow for explicit and direct single-column reordering without the need to select first:

Also, unrelated to this PR but related to reordering: I just noticed it's a bit annoying to not be able to create a new column selection starting from a currently selected column 😞 This behavior is inconsistent with body cell selection at the moment. Excel avoids this problem by enabling reordering when hovering the edge of the selection region, an interaction that has its own issues too (i.e. very thin hover target).

[giladgray]

weird issue where cursor changes to grabbing after drag is completed (or simply after a click without dragging)

[giladgray]

might be hard to grok, but here's a gif of my issue:

[cmslewis]

@llorca can definitely address your feedback in separate PRs! This PR is just a quick patch to fix wrongness in existing behavior.

[cmslewis]

@giladgray do my latest changes address the issues you were seeing?

[blueprint-bot]

Fix grabbing style when :active

Preview: documentation | table
Coverage: core | datetime

[giladgray]

looks great!