For creating a keyboard that does not resize the app window when it is shown we will require creating an APZ controller for overflow-y: scroll elements even if they are not overflowing yet. The reason for that is to make sure the user can bring into view some parts of the UI that may not be considered as scrollable since the window is not resized.

I assume such a thing will involve some hacking in layout/base/FrameLayerBuilder.cpp if someone wants to take a look at it.

Something like this.

I think the latest version of the patch in bug 970093 doesn't require this change. The root layer for the content process already has a scrollable metrics and APZ. If we just shrink the effective composition bounds of that layer (which is what the patch on bug 970093 does) then we shouldn't need to make any changes to FrameLayerBuilder or nsGfxScrollFrame.