Convert from line scrolling to pixel scrolling. Increase pixel scroll amount...

webkit/glue/webinputevent_win.cc

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 
 #include "webkit/glue/webinputevent.h"
 
 #include "base/logging.h"
 #include "base/string_util.h"
@@ skipping 177 matching lines @@
     modifiers |= CTRL_KEY;
   if (get_key_state(VK_MENU) & 0x8000)
     modifiers |= (ALT_KEY | META_KEY);
 
   // Set coordinates by translating event coordinates from screen to client.
   POINT client_point = { global_x, global_y };
   MapWindowPoints(NULL, hwnd, &client_point, 1);
   x = client_point.x;
   y = client_point.y;
 
-  // Convert wheel delta amount to a number of lines/chars to scroll.
+  // Convert wheel delta amount to a number of pixels to scroll.
+  //
+  // How many pixels should we scroll per line?  Gecko uses the height of the
+  // current line, which means scroll distance changes as you go through the
+  // page or go to different pages.  IE 7 is ~50 px/line, although the value
+  // seems to vary slightly by page and zoom level.  Since IE 7 has a smoothing
+  // algorithm on scrolling, it can get away with slightly larger scroll values
+  // without feeling jerky.  Here we use 100 px per three lines (the default
+  // scroll amount is three lines per wheel tick).
+  static const float kScrollbarPixelsPerLine = 100.0f / 3.0f;
   float scroll_delta = wheel_delta / WHEEL_DELTA;
   if (horizontal_scroll) {
     unsigned long scroll_chars = kDefaultScrollCharsPerWheelDelta;
     SystemParametersInfo(SPI_GETWHEELSCROLLCHARS, 0, &scroll_chars, 0);
-    scroll_delta *= static_cast<float>(scroll_chars);
+    // TODO(pkasting): Should probably have a different multiplier
+    // kScrollbarPixelsPerChar here.
+    scroll_delta *= static_cast<float>(scroll_chars) * kScrollbarPixelsPerLine;
   } else {
     unsigned long scroll_lines = kDefaultScrollLinesPerWheelDelta;
     SystemParametersInfo(SPI_GETWHEELSCROLLLINES, 0, &scroll_lines, 0);
     if (scroll_lines == WHEEL_PAGESCROLL)
       scroll_by_page = true;
-    if (!scroll_by_page)
-      scroll_delta *= static_cast<float>(scroll_lines);
+    if (!scroll_by_page) {
+      scroll_delta *=
+          static_cast<float>(scroll_lines) * kScrollbarPixelsPerLine;
+    }
   }
 
   // Set scroll amount based on above calculations.
   if (horizontal_scroll) {
-    // Scrolling up should move left, scrolling down should move right.  This is
-    // opposite Safari, but seems more consistent with vertical scrolling.
     delta_x = scroll_delta;
     delta_y = 0;
   } else {
     delta_x = 0;
     delta_y = scroll_delta;
   }
 }
 
 // WebKeyboardEvent -----------------------------------------------------------
 
@@ skipping 97 matching lines @@
     modifiers |= CTRL_KEY;
   if (GetKeyState(VK_MENU) & 0x8000)
     modifiers |= (ALT_KEY | META_KEY);
 
   if (LOWORD(lparam) > 1)
     modifiers |= IS_AUTO_REPEAT;
   if (IsKeyPad(wparam, lparam))
     modifiers |= IS_KEYPAD;
 }