The potential optimization described here is very difficult to implement correctly and is at odds with other work that has been done to optimize WebGL's display path. On macOS, WebGL-rendered canvases are drawn to the screen via Core Animation, which doesn't have any provisions for updating a sub-rectangle of an IOSurface-backed layer.

To implement this optimization it would be necessary to accumulate the scissor rectangles for all calls which affect the WebGL-rendered canvas in a given frame (draws, clears) and push the union of them through the compositing pipeline -- again, at odds with the direct display of WebGL-rendered canvases using the OS compositor, which has been shown to reduce copies and save power.

The basic assumption when using WebGL is that the underlying machine has the GPU horsepower to display at least a full-screen canvas with one or two passes of overdraw at the display's refresh rate. If this is true, then it should be possible to implement some optimization in the application -- like tiling of the backing store of the painting application into multiple smaller textures -- so that drawing operations can affect a reasonably small texture region, and re-display of the entire window can be fast, by blitting those tiles to the back buffer appropriately. The WebGL-rendered canvas should never be larger than the display -- the application should implement scrolling within a larger viewport itself.

Downgrading this to P3 and marking as a feature request. I don't anticipate we will implement this request in the near future.

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available. If you change it back, also remove the "Hotlist-Recharge-Cold" label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Leaving open, but this still seems like a P3 feature at best.