[webkit-dev] Scrolling overflow:hidden boxes..

Simon Fraser simon.fraser at apple.com
Fri Aug 16 15:26:24 PDT 2013

On Aug 16, 2013, at 2:45 PM, Antonio Gomes <tonikitoo at webkit.org> wrote:

> Hi.
> Bug https://bugs.webkit.org/show_bug.cgi?id=119760 (Text dragging can
> scroll overflow:hidden boxes) caught  my attention to the following
> situation:
> - imagine a page has an input field within a overflow:hidden <div>,
> and user starts text selecting from the input field text by dragging
> with the mouse. At some point it goes outside of the input boundary,
> reaches the outer <div> boundary. By then, 'autoscroll' takes place
> (see WebCore/page/AutoscrollController.cpp/h), and scrolls the outter
> <div>. In my opinion, it should not happen, due to div's
> overflow:hidden style.
> A natural solution for that could be hardening
> RenderLayer::scrollRectToVisible, when its upwards recursion occurs;
> so instead of picking the current layer parent, it picks the enclosing
> scrollable layer instead.
> However, it seems acceptable that overflow:hidden div's RenderLayers
> are  scrollable in certain situations. Consider the case of
> Element.scrollIntoView(), for example: as of now, WebKit scrolls an
> container overflow:hidden div, if it is to bring an HTML element into
> view.
> Both situations go through the same RenderLayer::scrollRectToVisible method.
> Introducing a ScrollSource parameter to indicate where the scrolling
> action came from (user interaction or not), and relax or harden the
> recursion criteria accordingly would not help, because it would break
> autoscrolling within shadow DOM content (think of autoscrolling an
> input's shadow DOM div).
> Do you guys have thoughts on that?

How do other browsers behave in terms of drag-scrolling of overflow:hidden, 
revealing content in overflow:hidden, and programmatic scrolling of overflow:hidden?


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