Long time request see https://github.com/sumatrapdfreader/sumatrapdf/issues/217
Where you may note this was the natural behaviour for a PDF reader long before browsers used a different technique. Double click has two functions one of which to select a word for inverse searching is not so obvious to the majority of users.
SumatraPDF behaviour is identical to Adobe Acrobat, Foxit Reader and Okular
The four main PDF viewer applications as used by dozens of LaTeX compilers.
Its not easy to please everyone, so some want from word to end of the line forwards, and some may want it to be whole paragraph, whilst that all depends on what the underlying language and text spacing allow.
For multiple lines or selecting a paragraph SumatraPDF has marquee selection using CTRL and left mouse. An extra feature not found in those others.
The code to just select one word was not easy and takes a few lines, see the number of issues about multiple word cut and paste “not working” because the underlying character definition is not standard
To do every desired permutation involves considerable, if this / then that deliberations.
I tried in the thread above a means to modify click behaviour which works 50% e.g. click word and drag to right is the easier one, but click and drag to left is not so easy. see https://github.com/sumatrapdfreader/sumatrapdf/issues/217#issuecomment-596605107.