"Apparently this problem has been around for years, and one bug report has been closed "won't fix". The issue has been reopened with Bug 11808 - Xfwm: Increase the resize border of window.
The width of the grab area is controlled by the theme." 1
Most people appear to circumvent the problem by using another means of resizing windows.
"As soon as your keyboard-shortcut (in Xfce upstream its Alt+F8) is hit, you can then resize the window by either moving your mouse (no need to click or drag) or use your keyboard arrow keys." 2
The width of the grab area is controlled by the theme." 1
Most people appear to circumvent the problem by using another means of resizing windows.
"As soon as your keyboard-shortcut (in Xfce upstream its Alt+F8) is hit, you can then resize the window by either moving your mouse (no need to click or drag) or use your keyboard arrow keys." 2
1 https://unix.stackexchange.com/questions/156435/how-can-i-ma...
2 https://xubuntu.org/news/window-resizing-in-xubuntu-and-xfce...