When using the mouse wheel button to zoom in and out, it may be more intuitive to maintain the drawing zoom center-point based on the cursor position. This can help orientation and prevent the drawing from "jumping" around while zooming in and out.

To do this I suggest the following implementation for ddisplay_zoom in display.c:

void
ddisplay_zoom(DDisplay *ddisp, Point *point, real magnify)
{
  Rectangle *visible;
  real width, height;
  // %%
  // cursor position ratios
  real rx,ry; 
  // %%

  visible = &ddisp->visible;

  // %%
  // ithai - calculate cursor position ratios
  rx = (point->x-visible->left)/(visible->right - visible->left);
  ry = (point->y-visible->top)/(visible->bottom - visible->top);
  // %%

  width = (visible->right - visible->left)/magnify;
  height = (visible->bottom - visible->top)/magnify;

  if ((ddisp->zoom_factor <= DDISPLAY_MIN_ZOOM) && (magnify<=1.0))
    return;
  if ((ddisp->zoom_factor >= DDISPLAY_MAX_ZOOM) && (magnify>=1.0))
    return;

  ddisp->zoom_factor *= magnify;

  // %%
  // ithai - set new origin based on the calculated ratios before zooming
  ddisplay_set_origo(ddisp, point->x-(width*rx),point->y-(height*ry));
  // %%
  
  ddisplay_update_scrollbars(ddisp);
  ddisplay_add_update_all(ddisp);
  ddisplay_flush(ddisp);

  update_zoom_status (ddisp);
}

This works better for me. Tested okay with v0.96+svn.