Dragging paintbrush off map causes highlighting to stay on

There's an issue with the drawing tools that can cause them to get into a broken state.

Steps to reproduce:
1. Load up the interface at https://districtbuilder.netlify.app/
2. Choose the paintbrush tool
3. Hold the mouse down to start selecting districts
4. Run the mouse off the map canvas (e.g. over the header) while still holding mouse button
5. Release mouse button

Expected result:
The selection is made and you have to click the mouse button again (over the map) to start selecting again.

Actual Result:
The drawing tool continues to select geounits after you return to the map without need the mouse button pressed down. I think you can escape this by clicking the mouse button, then accept/reject appears and clicking that breaks the selection mode.

Reproduce the issue at https://districtbuilder.netlify.app/ with the paintbrush and map canvas steps described. Trace the frontend handlers for dragging, leaving the canvas, and releasing the mouse, then verify that returning to the map does not continue selecting without a pressed button. Done means the reported reproduction no longer leaves the drawing tool in a broken state.

Written by the indexing model from the issue text.