STR:

1. Load JSON Viewer https://api.github.com/
2. Select the tree view text in the JSON tab
3. The selection disappears when mouse button is released -> BUG

AR:
The selection disappears when mouse button is released.

ER:
The selection persists when mouse button is released.

This bug can be easily reproduced anywhere where the TreeView component is used (e.g. in the Network panel -> Response panel showing formatted JSON preview)

https://searchfox.org/mozilla-central/rev/7d0c94a0e9a9fe1f83553f49b10128567d21709d/devtools/client/shared/components/tree/TreeView.js#89

Honza

One interesting thing is that the selection disappears only if the mouse cursor is not located on the selected text (only in the JSON viewer)
In case of the Response panel (in the Network panel) it always disappears.

Honza