Re: vertical scrollbar error on MS Windows

["grischka" <grishka@gmx.de>]

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Hi there,

Here is another fix for the scrollbars. From the approach
it calculates the thumb length once on mouse down and 
then leaves bar parameters alone during dragging. Patch 
for w32term.c is in the attachement.

Regards
-- grischka

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--- c:/checkout/emacs-cvs/emacs/src/w32term.c	Tue Feb 20 02:22:42 2007
+++ w32term.c	Wed Feb 21 00:10:48 2007
@@ -3468,11 +3468,70 @@
 /* Set the thumb size and position of scroll bar BAR.  We are currently
    displaying PORTION out of a whole WHOLE, and our position POSITION.  */
 
+#define SCROLL_FIX
+
 static void
 w32_set_scroll_bar_thumb (bar, portion, position, whole)
      struct scroll_bar *bar;
      int portion, position, whole;
 {
+#ifdef SCROLL_FIX
+    Window w;
+    BOOL draggingp = !NILP (bar->dragging) ? TRUE : FALSE;
+    SCROLLINFO si;
+    int sb_page, sb_pos, range, window, lines, n, mem;
+
+    static int was_draggingp;
+    if (draggingp && was_draggingp)
+        return;
+    was_draggingp = draggingp;
+
+    // scan some text
+    mem = whole;
+    if (mem > 20000)
+        mem = 20000;
+
+    for (lines = 0, n = 1; n <= mem; ++n)
+        if (FETCH_BYTE(n) == '\n')
+            ++lines;
+
+    window = WINDOW_TOTAL_LINES (XWINDOW (bar->window));
+    range = VERTICAL_SCROLL_BAR_INSIDE_HEIGHT (f, XINT(bar->height));
+
+    if (mem == whole) {
+        sb_page = range * window / (lines + window);
+    } else {
+        window = window * mem / (lines + 1); // in bytes, avg.
+        sb_page = range * window / (whole + window);
+    }
+
+    // bigger is nicer for them with a cheap mouse
+    n = VERTICAL_SCROLL_BAR_INSIDE_WIDTH (f, XINT(bar->width));
+    if (sb_page < n) {
+        sb_page = n;
+        // but not too big ...
+        n = range/4;
+        if (sb_page > n)
+            sb_page = n;
+    }
+
+    if (0 == whole)
+        sb_pos = 0;
+    else
+        sb_pos = ((double) position * (range - sb_page) + whole - 1) / whole;
+
+    si.cbSize = sizeof (si);
+    si.fMask = SIF_PAGE | SIF_POS | SIF_RANGE | SIF_DISABLENOSCROLL;
+    si.nMin = 0;
+    si.nMax = range - 1;
+    si.nPage = sb_page;
+    si.nPos = sb_pos;
+
+    w = SCROLL_BAR_W32_WINDOW (bar);
+    SetScrollInfo (w, SB_CTL, &si, TRUE);
+    return;
+
+#else
   Window w = SCROLL_BAR_W32_WINDOW (bar);
   /* We use the whole scroll-bar height in the calculations below, to
      avoid strange effects like scrolling backwards when just clicking
@@ -3538,6 +3597,7 @@
   SetScrollInfo (w, SB_CTL, &si, TRUE);
 
   UNBLOCK_INPUT;
+#endif
 }
 
 \f
@@ -3946,9 +4006,15 @@
     si.cbSize = sizeof (si);
     si.fMask = SIF_POS;
 
+#ifdef SCROLL_FIX
+    si.fMask = SIF_POS|SIF_RANGE|SIF_PAGE; //gr
+#endif
     GetScrollInfo ((HWND) msg->msg.lParam, SB_CTL, &si);
     y = si.nPos;
 
+#ifdef SCROLL_FIX
+    top_range = si.nMax + 1 - si.nPage; //gr
+#endif
     bar->dragging = Qnil;
 
 
@@ -3983,6 +4049,7 @@
 	bar->dragging = Qt;
 	emacs_event->part = scroll_bar_handle;
 
+#ifndef SCROLL_FIX
 	/* "Silently" update current position.  */
 	{
 	  SCROLLINFO si;
@@ -3996,11 +4063,13 @@
 
 	  SetScrollInfo (SCROLL_BAR_W32_WINDOW (bar), SB_CTL, &si, FALSE);
 	}
+#endif
 	break;
       case SB_ENDSCROLL:
 	/* If this is the end of a drag sequence, then reset the scroll
 	   handle size to normal and do a final redraw.  Otherwise do
 	   nothing.  */
+#ifndef SCROLL_FIX
 	if (dragging)
 	  {
 	    SCROLLINFO si;
@@ -4013,6 +4082,7 @@
 	    si.nPos = last_scroll_bar_drag_pos;
 	    SetScrollInfo (SCROLL_BAR_W32_WINDOW (bar), SB_CTL, &si, TRUE);
 	  }
+#endif
 	/* fall through */
       default:
 	emacs_event->kind = NO_EVENT;

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