Bug 804486 - Add toolkit.scrollbox.horizontalScrollDistance pref, default 5. Change horizontal scroll increment from 10px to horizontalScrollDistance * GetLineScrollAmount().width(). Make GetLineScrollAmount() use the font's AveCharWidth() for the horizontal amount rather than MaxHeight(). r=roc

content/events/src/nsEventStateManager.cpp

@@ -2820,8 +2820,7 @@ nsEventStateManager::GetScrollAmount(nsPresContext* aPresContext,
   nsLayoutUtils::GetFontMetricsForFrame(rootFrame, getter_AddRefs(fm),
     nsLayoutUtils::FontSizeInflationFor(rootFrame));
   NS_ENSURE_TRUE(fm, nsSize(0, 0));
-  int32_t fontHeight = fm->MaxHeight();
-  return nsSize(fontHeight, fontHeight);
+  return nsSize(fm->AveCharWidth(), fm->MaxHeight());
 }
 
 void

layout/base/nsPresShell.cpp

@@ -2215,7 +2215,9 @@ PresShell::ScrollCharacter(bool aRight)
   nsIScrollableFrame* scrollFrame =
     GetFrameToScrollAsScrollable(nsIPresShell::eHorizontal);
   if (scrollFrame) {
-    scrollFrame->ScrollBy(nsIntPoint(aRight ? 1 : -1, 0),
+    int32_t h = Preferences::GetInt("toolkit.scrollbox.horizontalScrollDistance",
+                                    NS_DEFAULT_HORIZONTAL_SCROLL_DISTANCE);
+    scrollFrame->ScrollBy(nsIntPoint(aRight ? h : -h, 0),
                           nsIScrollableFrame::LINES,
                           nsIScrollableFrame::SMOOTH);
   }

layout/generic/nsGfxScrollFrame.cpp

@@ -2385,13 +2385,12 @@ nsGfxScrollFrameInner::GetLineScrollAmount() const
                                 "mousewheel.min_line_scroll_amount", 1);
   }
   uint32_t appUnitsPerDevPixel = mOuter->PresContext()->AppUnitsPerDevPixel();
-  nscoord fontHeight =
+  nscoord minScrollAmountInAppUnits =
     NS_MAX(1, sMinLineScrollAmountInPixels) * appUnitsPerDevPixel;
-  if (fm) {
-    fontHeight = NS_MAX(fm->MaxHeight(), fontHeight);
-  }
-
-  return nsSize(fontHeight, fontHeight);
+  nscoord horizontalAmount = fm ? fm->AveCharWidth() : 0;
+  nscoord verticalAmount = fm ? fm->MaxHeight() : 0;
+  return nsSize(NS_MAX(horizontalAmount, minScrollAmountInAppUnits),
+                NS_MAX(verticalAmount, minScrollAmountInAppUnits));
 }
 
 /**
@@ -3422,17 +3421,19 @@ nsGfxScrollFrameInner::ReflowFinished()
   if (vScroll || hScroll) {
     nsWeakFrame weakFrame(mOuter);
     nsPoint scrollPos = GetScrollPosition();
-    // XXX shouldn't we use GetPageScrollAmount/GetLineScrollAmount here?
+    nsSize lineScrollAmount = GetLineScrollAmount();
     if (vScroll) {
-      const double kScrollMultiplier = Preferences::GetInt("toolkit.scrollbox.verticalScrollDistance",
-                                                           NS_DEFAULT_VERTICAL_SCROLL_DISTANCE);
-      nscoord fontHeight = GetLineScrollAmount().height * kScrollMultiplier;
+      const double kScrollMultiplier =
+        Preferences::GetInt("toolkit.scrollbox.verticalScrollDistance",
+                            NS_DEFAULT_VERTICAL_SCROLL_DISTANCE);
+      nscoord fontHeight = lineScrollAmount.height * kScrollMultiplier;
       // We normally use (scrollArea.height - fontHeight) for height
       // of page scrolling.  However, it is too small when
       // fontHeight is very large. (If fontHeight is larger than
       // scrollArea.height, direction of scrolling will be opposite).
       // To avoid it, we use (float(scrollArea.height) * 0.8) as
       // lower bound value of height of page scrolling. (bug 383267)
+      // XXX shouldn't we use GetPageScrollAmount here?
       nscoord pageincrement = nscoord(mScrollPort.height - fontHeight);
       nscoord pageincrementMin = nscoord(float(mScrollPort.height) * 0.8);
       FinishReflowForScrollbar(vScroll, minY, maxY, scrollPos.y,
@@ -3440,9 +3441,13 @@ nsGfxScrollFrameInner::ReflowFinished()
                                fontHeight);
     }
     if (hScroll) {
+      const double kScrollMultiplier =
+        Preferences::GetInt("toolkit.scrollbox.horizontalScrollDistance",
+                            NS_DEFAULT_HORIZONTAL_SCROLL_DISTANCE);
+      nscoord increment = lineScrollAmount.width * kScrollMultiplier;
       FinishReflowForScrollbar(hScroll, minX, maxX, scrollPos.x,
                                nscoord(float(mScrollPort.width) * 0.8),
-                               nsPresContext::CSSPixelsToAppUnits(10));
+                               increment);
     }
     NS_ENSURE_TRUE(weakFrame.IsAlive(), false);
   }

layout/generic/nsIScrollableFrame.h

@@ -14,7 +14,8 @@
 #include "nsCoord.h"
 #include "nsPresContext.h"
 
-#define NS_DEFAULT_VERTICAL_SCROLL_DISTANCE 3
+#define NS_DEFAULT_VERTICAL_SCROLL_DISTANCE   3
+#define NS_DEFAULT_HORIZONTAL_SCROLL_DISTANCE 5
 
 class nsBoxLayoutState;
 class nsIScrollPositionListener;

modules/libpref/src/init/all.js

@@ -342,8 +342,8 @@ pref("toolkit.autocomplete.richBoundaryCutoff", 200);
 
 pref("toolkit.scrollbox.smoothScroll", true);
 pref("toolkit.scrollbox.scrollIncrement", 20);
-// Make sure to update NS_DEFAULT_VERTICAL_SCROLL_DISTANCE if changing this default.
 pref("toolkit.scrollbox.verticalScrollDistance", 3);
+pref("toolkit.scrollbox.horizontalScrollDistance", 5);
 pref("toolkit.scrollbox.clickToScroll.scrollDelay", 150);
 
 // Telemetry