double-click-fuzz is a variable defined in `C source code'.
Its value is

Maximum mouse movement between clicks to make a double-click.
On window-system frames, value is the number of pixels the mouse may have
moved horizontally or vertically between two clicks to make a double-click.
On non window-system frames, value is interpreted in units of 1/8 characters
instead of pixels.

This variable is also the threshold for motion of the mouse
to count as a drag.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.