Ok got it working. Here is the code I'm using if anyone is wondering.
Inside the WndProc for hMapWin:
HWND hMainWin, hMapWin;
When window recieves focus:
After user is done scrolling (processed in both WM_VSCROLL and WM_HSCROLL)
ScreenToClient (hMapWin, &pMouse);
if ( pMouse.x < 585 && pMouse.y < 585)
Only problem I see is when using the arrows at the end of the scroll bar the focus switches between windows quickly if the user clicks the arrow insteads of holding it down. Oh well small price to pay