fix: improve selection of variable in workflow (#15484)

Signed-off-by: Yuichiro Utsumi <utsumi.yuichiro@fujitsu.com>

web/app/components/workflow/nodes/_base/components/variable/var-reference-vars.tsx

@@ -109,6 +109,7 @@ const Item: FC<ItemProps> = ({
             'relative w-full flex items-center h-6 pl-3  rounded-md cursor-pointer')
           }
           onClick={handleChosen}
+          onMouseDown={e => e.preventDefault()}
         >
           <div className='flex items-center w-0 grow'>
             {!isEnv && !isChatVar && <Variable02 className={cn('shrink-0 w-3.5 h-3.5 text-text-accent', isException && 'text-text-warning')} />}