When resizing a window, you must position your mouse over the resize thumb and hope that you are in the correct position when you click. If you are outside the thumb you will switch the active window (and possibly active app), if you are too far inside, nothing will happen. In either case you mutter, reposition and try again.
Most OS GUIs provide a feedback cursor change to indicate you are positioned over the resize thumb. Even OS X provides this for resizing columns (position the cursor over the left/right window divide in Finder), but not for windows.
Provide cursor feedback when over the thumb, as is already done for window columns. The open hand/grabbing hand would work nicely.