Inability to dock one tab when other is large.

[gwhitney]

Regarding docking. If you expand one pane at the side to be big, then when you drag the other pane over to the same side, there is no "blue drop zone" and you can't drop to dock. Instead if you drop, you end up floating over the docked pane. Do we want to allow dragging to dock in this situation, forcing resize of the large pane?

Originally posted by @katestange in https://github.com/numberscope/admin/discussions/6#discussioncomment-9970590

[gwhitney]

Planned per Oct 29 meeting:

• If you drag a tab so that your mouse pointer is over the sidebar, then it (always!) docks. If one position is occupied, it goes into the other position, even if it has to "warp" there. If neither position is occupied,it goes in top or bottom depending on where your mouse pointer ends up. (Make sure the mouse pointer glyph does not change while the tab is grabbed.) And in this case, it does not resize.
• If it is going into the "cavity" from one already there, then:
  • If the sum of the two sizes is less than the space available, leave the previously present one alone in size, and make the newly inserted one taller to fill up the space.
  • If the sum of the two sizes is greater than the space available, divvy up the whole space proportionally to the sizes before the drag.