Bug 54262 – Some Scrollbar functions assume an attached ScrollableArea but can be called without one

• Status: RESOLVED
• Resolution: FIXED
• Component: WebCore Misc.

[Peter Kasting]

EventHandler caches |m_lastScrollbarUnderMouse| while the user is interacting with a scrollbar.  This can cause problems if, during this interaction, the page goes through relayout and destroys its scrollbar.

Because |m_lastScrollbarUnderMouse| is a RefPtr, we don't need to worry about it being deleted.  However, in RenderLayer::destroyScrollbar(), scrollbar->disconnectFromScrollableArea() is called before the RenderLayer drops its ref.  As a result, the scrollbar no longer has an attached ScrollableArea.  The next time EventHandler calls e.g. Scrollbar::mouseMoved(), it blindly accesses scrollableArea() and we crash.

I don't know the model here well enough to know whether the right fix is for the Scrollbar to add NULL-checks in several places, or whether instead at the time the scrollbar is dropped from the RenderLayer the EventHandler should be told to drop |m_lastScrollbarUnderMouse|, or perhaps something else.

I don't have a minimal testcase that reproduces this bug, but I imagine a page that either continually flips between needing a scrollbar and not on a timer, or else one that turns of its scrollbar in response to onscroll, might be able to demonstrate this.  Setting up such a page and then grabbing the scroll thumb and dragging around might work.

[Peter Kasting]

(BTW, I may not have a _minimal_ testcase, but I do have one, locally, that's Chromium-specific, so I'm happy to test out any patches to fix.)

[James Robinson]

FYI this is a leading crasher on recent Chromium builds and does not seem restricted to any particular webpages (there are crash reports from youtube.com, facebook.com, google.*, etc).

[Sam Weinig]

I will add null checks to Scrollbar.  That is the correct way to fix this.  I should have a patch ready later today.

[Sam Weinig]

Created attachment 82268 [details]
Patch

[Sam Weinig]

Fixed in r78431.