avd opened the ticket and wrote:

Steps to reproduce:

1. Open a viewport
2. Drag the map in the viewport with the left mouse button
3. Move the mouse outside the viewport

When leaving the mouse outside the viewport, you should see that the map in the main window is moving.
Notice also that moving over an other window also changes the behavior of the mouse (drag is stopped when you move the mouse over them).

This tricks also works if you start dragging in the main window into a viewport.

I think a better behavior would be if the mouse 'remembers' the location where the drag started, which avoids these (at least for me) strange behavior.

Reported version: trunk
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/4419