Can confirm that this still happens in the latest build.

as of dev build 0.19355

it appears that any resizable repositionable window will display this behavior and it's not restricted to the area in the hotbar.

the currently active window's resizing and repositioning detection extends the entire game screen and is not restricted within the bounds of the window being adjusted.

1. open any window and hover the mouse cursor in line with the resize/reposition marker anywhere outside the windows boundary
2. click and drag

the window can be moved and resized without being within the bounds of the window.
some circumstances, you may be moving or resizing 2 windows at once depending if the adjustment markers are in alignment and the windows are from different groups, such as chat and inventory.

the crew, mail, and message windows will exhibit the same behavior but they seem to disallow any interaction outside of them, even amongst themselves. the last one opened of that group is the only thing you can interact with until closed, so any simultaneous movement of items in the hotbar and resizing of the window will only result in the window being affected and not the items in the hotbar.

the corner size adjustment seems to respect the boundaries of the window.

https://youtu.be/SN1NNvIpft8

version 0.19624

Just happened to me.