1238514 - Consider replacing the last use of usingDisplayPort in ScrollFrameHelper::BuildDisplayList with mWillBuildScrollableLayer

Status: RESOLVED DUPLICATE of bug 1405397

(Core :: Layout, defect)

Description

[Markus Stange [:mstange]]

There's only one use of usingDisplayPort left in ScrollFrameHelper::BuildDisplayList:

https://dxr.mozilla.org/mozilla-central/rev/6020a4cb41a77a09484c24a5875bb221714c0e6a/layout/generic/nsGfxScrollFrame.cpp#2974 :
> // If we are a root scroll frame that has a display port we want to add
> // scrollbars, they will be children of the scrollable layer, but they get
> // adjusted by the APZC automatically.
> bool addScrollBars = mIsRoot && usingDisplayPort && !aBuilder->IsForEventDelivery();

I don't really understand what this condition wants to achieve. Why not add scrollbars always? I suppose we don't want them to swallow touch events on B2G/Android, but I don't understand the rest of the conditions.

Comment 1

[Timothy Nikkel (:tnikkel)]

We did this in bug 1405397. https://hg.mozilla.org/mozilla-central/rev/d6fdc1d3b070