Dragging windows allows cursor to become detached from original location

I recently upgrade our project from 0.25 to 0.27 and noticed the dragging of windows now allows the mouse to be detached from the window being dragged, which I don't think is desirable as it results in awkward dragging.

I noticed that this is not unique to windows or our project, as you can easily replicate it in the web version (egui.rs) as well by just dragging windows into the side bars

Left: 0.25.0 -- right: 0.27.2

https://github.com/emilk/egui/assets/16842774/750fa65a-a0ec-4e4d-b5a9-1220cfdc4f2e

Reproduce the dragging behavior in the web version at egui.rs, especially when moving windows into the side bars, and compare it with the reported 0.25.0 and 0.27.2 behavior. Trace the window-dragging and pointer handling code; done means the cursor remains attached to the dragged window during this interaction.

Written by the indexing model from the issue text.