Import GTK WebInputEvent factory to content

content/browser/renderer_host/input/web_input_event_builders_gtk.cc

+// Copyright (c) 2013 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 "content/browser/renderer_host/input/web_input_event_builders_gtk.h"
+
+#include <gdk/gdk.h>
+#include <gdk/gdkkeysyms.h>
+#include <gtk/gtk.h>
+
+#include "base/logging.h"
+#include "content/browser/renderer_host/input/web_input_event_util_posix.h"
+#include "third_party/WebKit/public/web/WebInputEvent.h"
+#include "ui/base/keycodes/keyboard_code_conversion_gtk.h"
+
+using WebKit::WebInputEvent;
+using WebKit::WebMouseEvent;
+using WebKit::WebMouseWheelEvent;
+using WebKit::WebKeyboardEvent;
+
+namespace {
+
+// For click count tracking.
+static int num_clicks = 0;
+static GdkWindow* lack_click_event_window = 0;

[jdduke (slow), 2013/09/16 15:31:55]

nit: last_click_event_window

[garykac, 2013/09/16 22:45:05]

Done.

+static gint last_click_time = 0;
+static gint last_click_x = 0;
+static gint last_click_y = 0;
+static WebMouseEvent::Button last_click_button = WebMouseEvent::ButtonNone;
+
+bool ShouldForgetPreviousClick(GdkWindow* window, gint time, gint x, gint y) {
+  static GtkSettings* settings = gtk_settings_get_default();
+
+  if (window != lack_click_event_window)
+    return true;
+
+  gint double_click_time = 250;

[Seigo Nonaka, 2013/09/10 04:14:32]

nit: How about adding "const"?

[garykac, 2013/09/10 18:54:36]

This is passed by reference to g_object_get() below where the value can be
overwritten.

+  gint double_click_distance = 5;
+  g_object_get(G_OBJECT(settings),
+               "gtk-double-click-time",
+               &double_click_time,
+               "gtk-double-click-distance",
+               &double_click_distance,
+               NULL);
+  return (time - last_click_time) > double_click_time ||
+         std::abs(x - last_click_x) > double_click_distance ||
+         std::abs(y - last_click_y) > double_click_distance;
+}
+
+void ResetClickCountState() {

[jdduke (slow), 2013/09/16 15:31:55]

I realize this is a direct port from WebKit, but I wonder how necessary the
global state tracking is? Does it have to be process-wide, or would it be
sufficient to have some state/context stored per-RWHVGTK?

Anyhow, if that is appropriate (it may not be), it should probably happen in a
separate patch as this is more or less just moving code; perhaps we could file a
bug?

[garykac, 2013/09/16 22:45:05]

I added crbug.com/292733 to track this since I don't know the answer to it
offhand.

+  num_clicks = 0;
+  lack_click_event_window = 0;
+  last_click_time = 0;
+  last_click_x = 0;
+  last_click_y = 0;
+  last_click_button = WebKit::WebMouseEvent::ButtonNone;
+}
+
+bool IsKeyPadKeyval(guint keyval) {
+  // Keypad keyvals all fall into one range.
+  return keyval >= GDK_KP_Space && keyval <= GDK_KP_9;
+}
+
+double GdkEventTimeToWebEventTime(guint32 time) {
+  // Convert from time in ms to time in sec.
+  return time / 1000.0;
+}
+
+int GdkStateToWebEventModifiers(guint state) {
+  int modifiers = 0;
+  if (state & GDK_SHIFT_MASK)
+    modifiers |= WebInputEvent::ShiftKey;
+  if (state & GDK_CONTROL_MASK)
+    modifiers |= WebInputEvent::ControlKey;
+  if (state & GDK_MOD1_MASK)
+    modifiers |= WebInputEvent::AltKey;
+  if (state & GDK_META_MASK)
+    modifiers |= WebInputEvent::MetaKey;
+  if (state & GDK_BUTTON1_MASK)
+    modifiers |= WebInputEvent::LeftButtonDown;
+  if (state & GDK_BUTTON2_MASK)
+    modifiers |= WebInputEvent::MiddleButtonDown;
+  if (state & GDK_BUTTON3_MASK)
+    modifiers |= WebInputEvent::RightButtonDown;
+  if (state & GDK_LOCK_MASK)
+    modifiers |= WebInputEvent::CapsLockOn;
+  if (state & GDK_MOD2_MASK)
+    modifiers |= WebInputEvent::NumLockOn;
+  // Handle the AltGr (ISO Level3 Shift) key as Control + Alt.

[garykac, 2013/09/09 15:29:46]

nona: I added AltGr support here. I couldn't find docs that say GDK_MOD5_MASK is
AltGr, but determined it empirically.

[Seigo Nonaka, 2013/09/10 04:14:32]

Yes, Mod5Mask is for AltGr.
Thank you.

[garykac, 2013/09/10 18:54:36]

Thanks for the confirmation.

+  if (state & GDK_MOD5_MASK)
+    modifiers |= WebInputEvent::ControlKey | WebInputEvent::AltKey;
+  return modifiers;
+}
+
+ui::KeyboardCode GdkEventToWindowsKeyCode(const GdkEventKey* event) {
+  static const unsigned int kHardwareCodeToGDKKeyval[] = {
+    0,                 // 0x00:
+    0,                 // 0x01:
+    0,                 // 0x02:
+    0,                 // 0x03:
+    0,                 // 0x04:
+    0,                 // 0x05:
+    0,                 // 0x06:
+    0,                 // 0x07:
+    0,                 // 0x08:
+    0,                 // 0x09: GDK_Escape
+    GDK_1,             // 0x0A: GDK_1
+    GDK_2,             // 0x0B: GDK_2
+    GDK_3,             // 0x0C: GDK_3
+    GDK_4,             // 0x0D: GDK_4
+    GDK_5,             // 0x0E: GDK_5
+    GDK_6,             // 0x0F: GDK_6
+    GDK_7,             // 0x10: GDK_7
+    GDK_8,             // 0x11: GDK_8
+    GDK_9,             // 0x12: GDK_9
+    GDK_0,             // 0x13: GDK_0
+    GDK_minus,         // 0x14: GDK_minus
+    GDK_equal,         // 0x15: GDK_equal
+    0,                 // 0x16: GDK_BackSpace
+    0,                 // 0x17: GDK_Tab
+    GDK_q,             // 0x18: GDK_q
+    GDK_w,             // 0x19: GDK_w
+    GDK_e,             // 0x1A: GDK_e
+    GDK_r,             // 0x1B: GDK_r
+    GDK_t,             // 0x1C: GDK_t
+    GDK_y,             // 0x1D: GDK_y
+    GDK_u,             // 0x1E: GDK_u
+    GDK_i,             // 0x1F: GDK_i
+    GDK_o,             // 0x20: GDK_o
+    GDK_p,             // 0x21: GDK_p
+    GDK_bracketleft,   // 0x22: GDK_bracketleft
+    GDK_bracketright,  // 0x23: GDK_bracketright
+    0,                 // 0x24: GDK_Return
+    0,                 // 0x25: GDK_Control_L
+    GDK_a,             // 0x26: GDK_a
+    GDK_s,             // 0x27: GDK_s
+    GDK_d,             // 0x28: GDK_d
+    GDK_f,             // 0x29: GDK_f
+    GDK_g,             // 0x2A: GDK_g
+    GDK_h,             // 0x2B: GDK_h
+    GDK_j,             // 0x2C: GDK_j
+    GDK_k,             // 0x2D: GDK_k
+    GDK_l,             // 0x2E: GDK_l
+    GDK_semicolon,     // 0x2F: GDK_semicolon
+    GDK_apostrophe,    // 0x30: GDK_apostrophe
+    GDK_grave,         // 0x31: GDK_grave
+    0,                 // 0x32: GDK_Shift_L
+    GDK_backslash,     // 0x33: GDK_backslash
+    GDK_z,             // 0x34: GDK_z
+    GDK_x,             // 0x35: GDK_x
+    GDK_c,             // 0x36: GDK_c
+    GDK_v,             // 0x37: GDK_v
+    GDK_b,             // 0x38: GDK_b
+    GDK_n,             // 0x39: GDK_n
+    GDK_m,             // 0x3A: GDK_m
+    GDK_comma,         // 0x3B: GDK_comma
+    GDK_period,        // 0x3C: GDK_period
+    GDK_slash,         // 0x3D: GDK_slash
+    0,                 // 0x3E: GDK_Shift_R
+    0,                 // 0x3F:
+    0,                 // 0x40:
+    0,                 // 0x41:
+    0,                 // 0x42:
+    0,                 // 0x43:
+    0,                 // 0x44:
+    0,                 // 0x45:
+    0,                 // 0x46:
+    0,                 // 0x47:
+    0,                 // 0x48:
+    0,                 // 0x49:
+    0,                 // 0x4A:
+    0,                 // 0x4B:
+    0,                 // 0x4C:
+    0,                 // 0x4D:
+    0,                 // 0x4E:
+    0,                 // 0x4F:
+    0,                 // 0x50:
+    0,                 // 0x51:
+    0,                 // 0x52:
+    0,                 // 0x53:
+    0,                 // 0x54:
+    0,                 // 0x55:
+    0,                 // 0x56:
+    0,                 // 0x57:
+    0,                 // 0x58:
+    0,                 // 0x59:
+    0,                 // 0x5A:
+    0,                 // 0x5B:
+    0,                 // 0x5C:
+    0,                 // 0x5D:
+    0,                 // 0x5E:
+    0,                 // 0x5F:
+    0,                 // 0x60:
+    0,                 // 0x61:
+    0,                 // 0x62:
+    0,                 // 0x63:
+    0,                 // 0x64:
+    0,                 // 0x65:
+    0,                 // 0x66:
+    0,                 // 0x67:
+    0,                 // 0x68:
+    0,                 // 0x69:
+    0,                 // 0x6A:
+    0,                 // 0x6B:
+    0,                 // 0x6C:
+    0,                 // 0x6D:
+    0,                 // 0x6E:
+    0,                 // 0x6F:
+    0,                 // 0x70:
+    0,                 // 0x71:
+    0,                 // 0x72:
+    GDK_Super_L,       // 0x73: GDK_Super_L
+    GDK_Super_R,       // 0x74: GDK_Super_R
+  };
+
+  // |windows_key_code| has to include a valid virtual-key code even when we
+  // use non-US layouts, e.g. even when we type an 'A' key of a US keyboard
+  // on the Hebrew layout, |windows_key_code| should be VK_A.
+  // On the other hand, |event->keyval| value depends on the current
+  // GdkKeymap object, i.e. when we type an 'A' key of a US keyboard on
+  // the Hebrew layout, |event->keyval| becomes GDK_hebrew_shin and this
+  // ui::WindowsKeyCodeForGdkKeyCode() call returns 0.
+  // To improve compatibilty with Windows, we use |event->hardware_keycode|
+  // for retrieving its Windows key-code for the keys when the
+  // WebCore::windows_key_codeForEvent() call returns 0.
+  // We shouldn't use |event->hardware_keycode| for keys that GdkKeymap
+  // objects cannot change because |event->hardware_keycode| doesn't change
+  // even when we change the layout options, e.g. when we swap a control
+  // key and a caps-lock key, GTK doesn't swap their
+  // |event->hardware_keycode| values but swap their |event->keyval| values.
+  ui::KeyboardCode windows_key_code =
+      ui::WindowsKeyCodeForGdkKeyCode(event->keyval);
+  if (windows_key_code)
+    return windows_key_code;
+
+  if (event->hardware_keycode < arraysize(kHardwareCodeToGDKKeyval)) {
+    int keyval = kHardwareCodeToGDKKeyval[event->hardware_keycode];
+    if (keyval)
+      return ui::WindowsKeyCodeForGdkKeyCode(keyval);
+  }
+
+  // This key is one that keyboard-layout drivers cannot change.
+  // Use |event->keyval| to retrieve its |windows_key_code| value.
+  return ui::WindowsKeyCodeForGdkKeyCode(event->keyval);
+}
+
+// Normalizes event->state to make it Windows/Mac compatible. Since the way
+// of setting modifier mask on X is very different than Windows/Mac as shown
+// in http://crbug.com/127142#c8, the normalization is necessary.
+guint NormalizeEventState(const GdkEventKey* event) {
+  guint mask = 0;
+  switch (GdkEventToWindowsKeyCode(event)) {
+    case ui::VKEY_CONTROL:
+    case ui::VKEY_LCONTROL:
+    case ui::VKEY_RCONTROL:
+      mask = GDK_CONTROL_MASK;
+      break;
+    case ui::VKEY_SHIFT:
+    case ui::VKEY_LSHIFT:
+    case ui::VKEY_RSHIFT:
+      mask = GDK_SHIFT_MASK;
+      break;
+    case ui::VKEY_MENU:
+    case ui::VKEY_LMENU:
+    case ui::VKEY_RMENU:
+      mask = GDK_MOD1_MASK;
+      break;
+    case ui::VKEY_CAPITAL:
+      mask = GDK_LOCK_MASK;
+      break;
+    default:
+      return event->state;
+  }
+  if (event->type == GDK_KEY_PRESS)
+    return event->state | mask;
+  return event->state & ~mask;
+}
+
+// Gets the corresponding control character of a specified key code. See:
+// http://en.wikipedia.org/wiki/Control_characters
+// We emulate Windows behavior here.
+int GetControlCharacter(ui::KeyboardCode windows_key_code, bool shift) {
+  if (windows_key_code >= ui::VKEY_A && windows_key_code <= ui::VKEY_Z) {
+    // ctrl-A ~ ctrl-Z map to \x01 ~ \x1A
+    return windows_key_code - ui::VKEY_A + 1;
+  }
+  if (shift) {
+    // following graphics chars require shift key to input.
+    switch (windows_key_code) {
+      // ctrl-@ maps to \x00 (Null byte)
+      case ui::VKEY_2:
+        return 0;
+      // ctrl-^ maps to \x1E (Record separator, Information separator two)
+      case ui::VKEY_6:
+        return 0x1E;
+      // ctrl-_ maps to \x1F (Unit separator, Information separator one)
+      case ui::VKEY_OEM_MINUS:
+        return 0x1F;
+      // Returns 0 for all other keys to avoid inputting unexpected chars.
+      default:
+        return 0;
+    }
+  } else {
+    switch (windows_key_code) {
+      // ctrl-[ maps to \x1B (Escape)
+      case ui::VKEY_OEM_4:
+        return 0x1B;
+      // ctrl-\ maps to \x1C (File separator, Information separator four)
+      case ui::VKEY_OEM_5:
+        return 0x1C;
+      // ctrl-] maps to \x1D (Group separator, Information separator three)
+      case ui::VKEY_OEM_6:
+        return 0x1D;
+      // ctrl-Enter maps to \x0A (Line feed)
+      case ui::VKEY_RETURN:
+        return 0x0A;
+      // Returns 0 for all other keys to avoid inputting unexpected chars.
+      default:
+        return 0;
+    }
+  }
+}
+
+}  // namespace
+
+namespace content {
+
+// WebKeyboardEvent -----------------------------------------------------------
+
+WebKeyboardEvent WebKeyboardEventBuilder::Build(const GdkEventKey* event) {
+  WebKeyboardEvent result;
+
+  result.timeStampSeconds = GdkEventTimeToWebEventTime(event->time);
+  result.modifiers = GdkStateToWebEventModifiers(NormalizeEventState(event));
+
+  switch (event->type) {
+    case GDK_KEY_RELEASE:
+      result.type = WebInputEvent::KeyUp;
+      break;
+    case GDK_KEY_PRESS:
+      result.type = WebInputEvent::RawKeyDown;
+      break;
+    default:
+      NOTREACHED();
+  }
+
+  // According to MSDN:
+  // http://msdn.microsoft.com/en-us/library/ms646286(VS.85).aspx
+  // Key events with Alt modifier and F10 are system key events.
+  // We just emulate this behavior. It's necessary to prevent webkit from
+  // processing keypress event generated by alt-d, etc.
+  // F10 is not special on Linux, so don't treat it as system key.
+  if (result.modifiers & WebInputEvent::AltKey)
+    result.isSystemKey = true;
+
+  // The key code tells us which physical key was pressed (for example, the
+  // A key went down or up).  It does not determine whether A should be lower
+  // or upper case.  This is what text does, which should be the keyval.
+  ui::KeyboardCode windows_key_code = GdkEventToWindowsKeyCode(event);
+  result.windowsKeyCode = GetWindowsKeyCodeWithoutLocation(windows_key_code);
+  result.modifiers |= GetLocationModifiersFromWindowsKeyCode(windows_key_code);
+  result.nativeKeyCode = event->hardware_keycode;
+
+  if (result.windowsKeyCode == ui::VKEY_RETURN) {
+    // We need to treat the enter key as a key press of character \r.  This
+    // is apparently just how webkit handles it and what it expects.
+    result.unmodifiedText[0] = '\r';
+  } else {
+    // FIXME: fix for non BMP chars
+    result.unmodifiedText[0] =
+        static_cast<int>(gdk_keyval_to_unicode(event->keyval));
+  }
+
+  // If ctrl key is pressed down, then control character shall be input.
+  if (result.modifiers & WebInputEvent::ControlKey) {
+    result.text[0] =
+      GetControlCharacter(ui::KeyboardCode(result.windowsKeyCode),
+                          result.modifiers & WebInputEvent::ShiftKey);
+  } else {
+    result.text[0] = result.unmodifiedText[0];
+  }
+
+  result.setKeyIdentifierFromWindowsKeyCode();
+
+  // FIXME: Do we need to set IsAutoRepeat?
+  if (IsKeyPadKeyval(event->keyval))
+    result.modifiers |= WebInputEvent::IsKeyPad;
+
+  return result;
+}
+
+WebKeyboardEvent WebKeyboardEventBuilder::Build(wchar_t character,
+                                                     int state,
+                                                     double timeStampSeconds) {
+  // keyboardEvent(const GdkEventKey*) depends on the GdkEventKey object and
+  // it is hard to use/ it from signal handlers which don't use GdkEventKey
+  // objects (e.g. GtkIMContext signal handlers.) For such handlers, this
+  // function creates a WebInputEvent::Char event without using a
+  // GdkEventKey object.
+  WebKeyboardEvent result;
+  result.type = WebKit::WebInputEvent::Char;
+  result.timeStampSeconds = timeStampSeconds;
+  result.modifiers = GdkStateToWebEventModifiers(state);
+  result.windowsKeyCode = character;
+  result.nativeKeyCode = character;
+  result.text[0] = character;
+  result.unmodifiedText[0] = character;
+
+  // According to MSDN:
+  // http://msdn.microsoft.com/en-us/library/ms646286(VS.85).aspx
+  // Key events with Alt modifier and F10 are system key events.
+  // We just emulate this behavior. It's necessary to prevent webkit from
+  // processing keypress event generated by alt-d, etc.
+  // F10 is not special on Linux, so don't treat it as system key.
+  if (result.modifiers & WebInputEvent::AltKey)
+    result.isSystemKey = true;
+
+  return result;
+}
+
+// WebMouseEvent --------------------------------------------------------------
+
+WebMouseEvent WebMouseEventBuilder::Build(const GdkEventButton* event) {
+  WebMouseEvent result;
+
+  result.timeStampSeconds = GdkEventTimeToWebEventTime(event->time);
+
+  result.modifiers = GdkStateToWebEventModifiers(event->state);
+  result.x = static_cast<int>(event->x);
+  result.y = static_cast<int>(event->y);
+  result.windowX = result.x;
+  result.windowY = result.y;
+  result.globalX = static_cast<int>(event->x_root);
+  result.globalY = static_cast<int>(event->y_root);
+  result.clickCount = 0;
+
+  switch (event->type) {
+    case GDK_BUTTON_PRESS:
+      result.type = WebInputEvent::MouseDown;
+      break;
+    case GDK_BUTTON_RELEASE:
+      result.type = WebInputEvent::MouseUp;
+      break;
+    case GDK_3BUTTON_PRESS:
+    case GDK_2BUTTON_PRESS:

[garykac, 2013/09/09 15:29:46]

nona: double- and triple-clicks are handled below (in result.clickCount), rather
than relying on these extra GDK events.

[Seigo Nonaka, 2013/09/10 04:14:32]

Sure, make sense.

+    default:
+      NOTREACHED();
+  }
+
+  result.button = WebMouseEvent::ButtonNone;
+  if (event->button == 1)
+    result.button = WebMouseEvent::ButtonLeft;
+  else if (event->button == 2)
+    result.button = WebMouseEvent::ButtonMiddle;
+  else if (event->button == 3)
+    result.button = WebMouseEvent::ButtonRight;
+
+  if (result.type == WebInputEvent::MouseDown) {
+    bool forgetPreviousClick = ShouldForgetPreviousClick(
+        event->window, event->time, event->x, event->y);
+
+    if (!forgetPreviousClick && result.button == last_click_button) {
+      ++num_clicks;
+    } else {
+      num_clicks = 1;
+
+      lack_click_event_window = event->window;
+      last_click_x = event->x;
+      last_click_y = event->y;
+      last_click_button = result.button;
+    }
+    last_click_time = event->time;
+  }
+  result.clickCount = num_clicks;
+
+  return result;
+}
+
+WebMouseEvent WebMouseEventBuilder::Build(const GdkEventMotion* event) {
+  WebMouseEvent result;
+
+  result.timeStampSeconds = GdkEventTimeToWebEventTime(event->time);
+  result.modifiers = GdkStateToWebEventModifiers(event->state);
+  result.x = static_cast<int>(event->x);
+  result.y = static_cast<int>(event->y);
+  result.windowX = result.x;
+  result.windowY = result.y;
+  result.globalX = static_cast<int>(event->x_root);
+  result.globalY = static_cast<int>(event->y_root);
+
+  switch (event->type) {
+    case GDK_MOTION_NOTIFY:
+      result.type = WebInputEvent::MouseMove;
+      break;
+    default:
+      NOTREACHED();
+  }
+
+  result.button = WebMouseEvent::ButtonNone;
+  if (event->state & GDK_BUTTON1_MASK)
+    result.button = WebMouseEvent::ButtonLeft;
+  else if (event->state & GDK_BUTTON2_MASK)
+    result.button = WebMouseEvent::ButtonMiddle;
+  else if (event->state & GDK_BUTTON3_MASK)
+    result.button = WebMouseEvent::ButtonRight;
+
+  if (ShouldForgetPreviousClick(event->window, event->time, event->x, event->y))
+    ResetClickCountState();
+
+  return result;
+}
+
+WebMouseEvent WebMouseEventBuilder::Build(const GdkEventCrossing* event) {
+  WebMouseEvent result;
+
+  result.timeStampSeconds = GdkEventTimeToWebEventTime(event->time);
+  result.modifiers = GdkStateToWebEventModifiers(event->state);
+  result.x = static_cast<int>(event->x);
+  result.y = static_cast<int>(event->y);
+  result.windowX = result.x;
+  result.windowY = result.y;
+  result.globalX = static_cast<int>(event->x_root);
+  result.globalY = static_cast<int>(event->y_root);
+
+  switch (event->type) {
+    case GDK_ENTER_NOTIFY:
+    case GDK_LEAVE_NOTIFY:
+      // Note that if we sent MouseEnter or MouseLeave to WebKit, it
+      // wouldn't work - they don't result in the proper JavaScript events.
+      // MouseMove does the right thing.
+      result.type = WebInputEvent::MouseMove;
+      break;
+    default:
+      NOTREACHED();
+  }
+
+  result.button = WebMouseEvent::ButtonNone;
+  if (event->state & GDK_BUTTON1_MASK)
+    result.button = WebMouseEvent::ButtonLeft;
+  else if (event->state & GDK_BUTTON2_MASK)
+    result.button = WebMouseEvent::ButtonMiddle;
+  else if (event->state & GDK_BUTTON3_MASK)
+    result.button = WebMouseEvent::ButtonRight;
+
+  if (ShouldForgetPreviousClick(event->window, event->time, event->x, event->y))
+    ResetClickCountState();
+
+  return result;
+}
+
+// WebMouseWheelEvent ---------------------------------------------------------
+
+WebMouseWheelEvent WebMouseWheelEventBuilder::Build(
+    const GdkEventScroll* event) {
+  WebMouseWheelEvent result;
+
+  result.type = WebInputEvent::MouseWheel;
+  result.button = WebMouseEvent::ButtonNone;
+
+  result.timeStampSeconds = GdkEventTimeToWebEventTime(event->time);
+  result.modifiers = GdkStateToWebEventModifiers(event->state);
+  result.x = static_cast<int>(event->x);
+  result.y = static_cast<int>(event->y);
+  result.windowX = result.x;
+  result.windowY = result.y;
+  result.globalX = static_cast<int>(event->x_root);
+  result.globalY = static_cast<int>(event->y_root);
+
+  // How much should we scroll per mouse wheel event?
+  // - Windows uses 3 lines by default and obeys a system setting.
+  // - Mozilla has a pref that lets you either use the "system" number of lines
+  //   to scroll, or lets the user override it.
+  //   For the "system" number of lines, it appears they've hardcoded 3.
+  //   See case NS_MOUSE_SCROLL in content/events/src/nsEventStateManager.cpp
+  //   and InitMouseScrollEvent in widget/src/gtk2/nsCommonWidget.cpp .
+  // - Gtk makes the scroll amount a function of the size of the scroll bar,
+  //   which is not available to us here.
+  // Instead, we pick a number that empirically matches Firefox's behavior.
+  static const float scrollbarPixelsPerTick = 160.0f / 3.0f;

[jdduke (slow), 2013/09/16 23:46:58]

Here's the other use I was thinking of.

+
+  switch (event->direction) {
+    case GDK_SCROLL_UP:
+      result.deltaY = scrollbarPixelsPerTick;
+      result.wheelTicksY = 1;
+      break;
+    case GDK_SCROLL_DOWN:
+      result.deltaY = -scrollbarPixelsPerTick;
+      result.wheelTicksY = -1;
+      break;
+    case GDK_SCROLL_LEFT:
+      result.deltaX = scrollbarPixelsPerTick;
+      result.wheelTicksX = 1;
+      break;
+    case GDK_SCROLL_RIGHT:
+      result.deltaX = -scrollbarPixelsPerTick;
+      result.wheelTicksX = -1;
+      break;
+  }
+
+  return result;
+}
+
+}  // namespace content