improve dropzones

[sgratzl]

closes #504

prerequisites:

• branch is up-to-date with the branch to be merged with, i.e. develop
• build is successful
• code is cleaned up and formatted

Summary

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[alexsb]

Awesome, thanks!

Two comments though:

1. The "place" indicator is as wide as it's drop zone (1/3) in one column, but should probably extend to the next column over as well, as it really doesn't matter whether I'm on the right 1/3 of column A or the left 1/3 of column B.
2. Once a "sum" indicator appears, it's not possible to get the "place" indicator back for that column, unless you move off the column. We could still have the three drop zones aktive, even if the "sum" indicator is visible, to address this. See the problem here:
   https://user-images.githubusercontent.com/2039375/148050998-29a508db-85c5-4653-a734-fc51af783186.mov

[sgratzl]

current version:

[alexsb]

This is awesome! Works really nice.

One more suggestion: it might be useful to render the target action (place here, sum, etc.) also (or only?) in the header that you're dragging. You can easily get into situations where the label isn't readable otherwise:

[sgratzl]

the header image is rendered by the browser not by me, thus not sure whether I can manipulate it on the fly. Even if it would be to manipulate the original header of the column being dragged within the LineUp header row. Thus, it might be more confusing that helpful.

[alexsb]

OK, I don't think it's critical, so good to merge as far as I'm concerned.

[puehringer]

One thing I noticed: when moving a column to the right next dropzone, the column of the right neighbor is rendered above the zone.

[puehringer]

One thing I noticed: when moving a column to the right next dropzone, the column of the right neighbor is rendered above the zone.

Any update on this issue?

[sgratzl]

One thing I noticed: when moving a column to the right next dropzone, the column of the right neighbor is rendered above the zone.

[puehringer]

Works for me now as expected, I think this PR can be merged as of now 🎉