Prevent AutoscrollForSelection when selecting text in a fixed-position element.

third_party/WebKit/Source/core/layout/LayoutBox.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBox.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBox.cpp b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
index a240f8a112be65e8281829b5e91e3cc0048964ac..2d8f951adcbe93d9c152b6404f1c878127c7013e 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
@@ -1008,6 +1008,12 @@ LayoutBox* LayoutBox::findAutoscrollable(LayoutObject* layoutObject) {
   while (
       layoutObject &&
       !(layoutObject->isBox() && toLayoutBox(layoutObject)->canAutoscroll())) {
+    // Do not start autoscroll when the node is inside a fixed-position element.
+    if (layoutObject->isBox() && toLayoutBox(layoutObject)->hasLayer() &&

[bokan, 2016/10/24 13:22:00]

Could you confirm that a scroller inside a position: fixed can still be auto
scrolled? e.g.

<div style="position: fixed">
  <div style="overflow: scroll">
    Selecting this text should allow autoscrolling within the overflow div
  </div>
</div>

[sunyunjia, 2016/10/24 14:10:25]

Confirmed!

+        toLayoutBox(layoutObject)->layer()->scrollsWithViewport()) {
+      return 0;
+    }
+
     if (!layoutObject->parent() &&
         layoutObject->node() == layoutObject->document() &&
         layoutObject->document().localOwner())