Take into account the scrollbar in page overlap

When the horizontal scrollbar is visible the viewport size is reduced,
so the page overlap should be computed from the visible viewport only.
The change ensures the overlap has the same lenght, regardless of the
visibility state of the scrollbars.

dw/fltkviewport.cc

@@ -526,14 +526,16 @@ void FltkViewport::scroll (int dx, int dy)
 
 void FltkViewport::scroll (core::ScrollCommand cmd)
 {
+   int hdiff = vscrollbar->visible () ? SCROLLBAR_THICKNESS : 0;
+   int vdiff = hscrollbar->visible () ? SCROLLBAR_THICKNESS : 0;
    if (cmd == core::SCREEN_UP_CMD) {
-      scroll (0, -h () + pageOverlap);
+      scroll (0, -h () + pageOverlap + vdiff);
    } else if (cmd == core::SCREEN_DOWN_CMD) {
-      scroll (0, h () - pageOverlap);
+      scroll (0, h () - pageOverlap - vdiff);
    } else if (cmd == core::SCREEN_LEFT_CMD) {
-      scroll (-w() + pageOverlap, 0);
+      scroll (-w() + pageOverlap + hdiff, 0);
    } else if (cmd == core::SCREEN_RIGHT_CMD) {
-      scroll (w() - pageOverlap, 0);
+      scroll (w() - pageOverlap - hdiff, 0);
    } else if (cmd == core::LINE_UP_CMD) {
       scroll (0, -vscrollbar->linesize ());
    } else if (cmd == core::LINE_DOWN_CMD) {