In bug 1682365 we switched from storing picture cache tiles in texture arrays, to using individual 2d textures for each tile. This is because we want to move away from texture arrays due to many issues we have encountered with them. The downside is that on platforms without a native OS compositor we now need to use a draw call per tile when compositing, rather than being able to batch those in the same texture together.

This probably isn't a huge issue, as the number of tiles we are compositing will be quite small. But it would be good at some point to store multiple tiles in a larger 2d texture. Previously we used a 1024x512x8 2d texture array, so switching to a 2048x2048 2d texture would be equivalent. I'm not sure what size would be best for the smaller scrollbar picture slices.