wxScrolledWindow::GetViewStart

void GetViewStart(int* x, int* y) const
Get the position at which the visible portion of the window starts.

Parameters
x　Receives the first visible x position in scroll units.
y　Receives the first visible y position in scroll units.

Remarks
If either of the scrollbars is not at the home position,
x and/or y will be greater than zero.
Combined with wxWindow::GetClientSize,
the application can use this function to efficiently redraw only the visible portion of the window.
The positions are in logical scroll units,
not pixels, so to convert to pixels you will have to multiply by the number of pixels per scroll increment.