Why is panel layout such a special-case concept in so many programs?

It's almost like you'd want some kind of underlying UI substrate or mechanism... provided by some kind of... hm... opera- yes! OPERATING SYSTEM... perhaps made by a company called MICROSOFT... which natively provides tab/panel/windowing mechanisms that provides what everyone implements separately today!!!

Am I not seeing this right or did it take a half second for the sidebar to change sides in their video? lol So how many cases do you need to handle for panel movement? Is it just drop in window, drop between windows, or drop into tab list?

You are seeing that correctly, and it was posted without anyone noticing :( In RADDBG the panels are implemented as an n-ary tree. Each set of children subdivide some space on one axis. For drag/drop, you have a number of drop sites, which just encode a move from one spot in the n-ary tree to another. You have fine-grained ones, like dropping onto an existing panel, or into a tab bar, or you have "splitting" ones, which exist between boundaries between existing nodes in the tree.

Give them a break—it's JS! It's like Doom in PDF, but you realize you don't know what Doom or PDF is by the time you're done.

They somehow manage to make it worse than what they did with Visual Studio (no Code)

The sad thing is that (the "real") Visual Studio does this correctly for decades now.

If by "correctly" you mean "crashes when you drag a panel around, and only some of the panels are draggable", then... I guess?

it also takes like a full second for vscode to changes sides lol

What.. what is this? Looks so good.