1201336 - [meta] Make background-position updates cheaper to repaint

Status: NEW

(Core :: Layout, defect)

Description

[Markus Stange [:mstange]]

This will help with bug 1201231.

Comment 1

[Daniel Holbert [:dholbert]]

Can you elaborate on what's unnecessarily-slow about them, and/or where the opportunity is for making things cheaper?

Comment 2

[Markus Stange [:mstange]]

They cause us to repaint too much and too slowly. Too much because we're (1) putting background images in the same layer as the rest of the page, even when background-position is animated (so we need to repaint all the layer content that intersects with the image) and (2) because we're repainting the whole frame subtree starting at the frame with the updated background-position (because of the RepaintFrame change hint). Too slowly because we're painting them into PaintedLayers instead of using ImageLayers for them, which is bad on all platforms that have unaccelerated main thread painting and accelerated compositing.

See the dependent bugs for possible remedies.