Chromium 77 changes to getCanvasData breaks selection performance.

Describe the bug Chromium changes to getCanvasData (which drives forEachFeatureAtPixel) has reduced performance of OpenLayers selection by 50-100x

To Reproduce Steps to reproduce the behavior:

1. Open Chrome 77
2. Play with the Select Features example and monitor performance (https://openlayers.org/en/latest/examples/select-features.html)

Expected behavior 50-100x faster performance in my experience. My personal app now takes 800ms to select a feature.

Discussion https://bugs.chromium.org/p/chromium/issues/detail?id=1001845

It looks like the Chromium team might try to mitigate this, but they have also indicated it was known and intentional. It’s possible we need to re-work how OpenLayers does selection. Is there a way to limit how many calls to getCanvasData it has to make? I see it making dozens for a single selection. I imagine this is related to testing against many candidate features in the r-tree?

Workarounds You can disable 2D Canvas Acceleration in Chrome via chrome://flags

[ablakey]

Looks like they’re pretty committed to fixing this and next release sounds probable: https://bugs.chromium.org/p/chromium/issues/detail?id=1001845#c14

[ahocevar]

@nikess Should be done in a week or so.