[Drag & drop, intro] Improve the drop target line

[Witoso]

We want to improve the drop target line for both horizontal and vertical lines:

• Symmetrical approach – arrow should also be added in the horizontal line, 
  • RTL, the arrow should be on the other side.
• For darker background
  • The lines vertical and horizontal by default should be surrounded by the white stroke (0.5-1px).
  • Outline may be tricky to style, intersections, etc., SVG?
  • Possible scope cut.
• I should be possible to style both line color and the surrounding stroke by the CSS.

Internal Figma file.

If you'd like to see this improvement implemented, add a 👍 reaction to this post.

[Witoso]

@dagdzi very worthwhile comment from @niegowski, having half-pixels could look messy (antialiasing, and other subpixel rendering magic), and it would be better to use full pixels.

[dagdzi]

@Witoso In that case, I'd stick with the 2 px line. I think it will be more visible and consistent with the rest of the interface elements.