Question

OS X Lion and above allow the user to turn on/off iOS-style floating scroll bars – either manually via System Preferences, or by plugging in a mouse.

WebKit-based browsers (and maybe Opera?) switch their scroll bar style immediately – is there an event that fires when this occurs? (WebKit-only is just fine)

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Some notes:

• OS X fires NSPreferredScrollerStyleDidChangeNotification when the user switches their scroll bar style

• WebKit does not appear to subscribe to this event (no hits when grepping for it).

• I suspect that WebKit is handling this via the NSViewBoundsDidChangeNotification event (which, I assume, fires for the scroll view’s content view).

• WebKit handles this event within -[WebHTMLView _frameOrBoundsChanged], which seems like one potential re-layouting point.

• There are also references to this notification in WebPDFView and the inspector’s WebNodeHighlight – both seem unrelated to this case.

• -[WebDynamicScrollBarsView adjustForScrollOriginChange] seems to indicate that events may not be fired (if so, it’d be nice to see some confirmation)

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Polling for changes is not an acceptable answer to me (performance & the layout jumping after the user changes the value).

Answer 1

From everything I’ve read thus far, there is no DOM event fired during this style transition.

The most conclusive example is to create an overflow: auto div, and then log all of its events (in Safari or Chrome) via monitorEvents. Nothing is logged when changing the scroll bar style.

I have an open webkit bug on the issue (though it’s probably minor enough that we won’t see any movement on it).