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 (click to see):