When scrolling part of the window, scroll the update region as well so that

parts that haven't been painted yet still get an update event in their new
location.  This code seems far too complicated, and I suspect there's a
better way.  Still, scrolling continuously with the window partially
off-screen now works.

[originally from svn r2284]

mac/macterm.c

@@ -1,4 +1,4 @@
-/* $Id: macterm.c,v 1.16 2002/11/29 00:32:03 ben Exp $ */
+/* $Id: macterm.c,v 1.17 2002/12/07 15:21:56 ben Exp $ */
 /*
  * Copyright (c) 1999 Simon Tatham
  * Copyright (c) 1999, 2002 Ben Harris
@@ -1269,20 +1269,46 @@ void palette_reset(void *frontend) {
 void do_scroll(void *frontend, int topline, int botline, int lines) {
     Session *s = frontend;
     Rect r;
-    RgnHandle update;
+    RgnHandle scrollrgn = NewRgn();
+    RgnHandle movedupdate = NewRgn();
+    RgnHandle update = NewRgn();
+    Point g2l = { 0, 0 };
 
     SetPort(s->window);
+
+    /*
+     * Work out the part of the update region that will scrolled by
+     * this operation.
+     */
+    if (lines > 0)
+	SetRectRgn(scrollrgn, 0, (topline + lines) * s->font_height,
+		   s->term->cols * s->font_width,
+		   (botline + 1) * s->font_height);
+    else
+	SetRectRgn(scrollrgn, 0, topline * s->font_height,
+		   s->term->cols * s->font_width,
+		   (botline - lines + 1) * s->font_height);
+    CopyRgn(((WindowPeek)s->window)->updateRgn, movedupdate);
+    GlobalToLocal(&g2l);
+    OffsetRgn(movedupdate, g2l.h, g2l.v); /* Convert to local co-ords. */
+    SectRgn(scrollrgn, movedupdate, movedupdate); /* Clip scrolled section. */
+    ValidRgn(movedupdate);
+    OffsetRgn(movedupdate, 0, -lines * s->font_height); /* Scroll it. */
+
     PenNormal();
     if (HAVE_COLOR_QD())
 	PmBackColor(DEFAULT_BG);
     else
 	BackColor(blackColor); /* XXX make configurable */
-    update = NewRgn();
     SetRect(&r, 0, topline * s->font_height,
 	    s->term->cols * s->font_width, (botline + 1) * s->font_height);
     ScrollRect(&r, 0, - lines * s->font_height, update);
-    /* XXX: move update region? */
+
     InvalRgn(update);
+    InvalRgn(movedupdate);
+
+    DisposeRgn(scrollrgn);
+    DisposeRgn(movedupdate);
     DisposeRgn(update);
 }