Operating system: Windows-8.1, Windows-10, Linux Mint, Archlinux
Graphics card: any
Broken: version: 2.81, 2.80
**Short description of error**
Left click don't select vertex during mouse movement - mouse have to be strightly still to select vertex on left click.
**Exact steps for others to reproduce the error**
Enter edit mode of default cube, switch selection tool from "select box" to "tweak", select vertex, try to add vertices to selection with shift+LMB and short dragging.
Blender 2.79 behaviour, (or 2.80 - 2.81 Right click select behaviour, it also works ok):
Blender 2.80 - 2.81 Left Click select behaviour: