composite_simple() calculate combined dirty rect from all tile's dirty rect. But the combined dirty rect becomes invalid when there is a tile that is not used that was used before. https://searchfox.org/mozilla-central/rev/7e92a667e3829831c31e8d46aefe7ef67ad5be1c/gfx/wr/webrender/src/renderer.rs#4292
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composite_simple() calculates combined dirty rect from all tile's dirty rect. But the combined dirty rect becomes invalid when there is a tile that is not used that was used before. https://searchfox.org/mozilla-central/rev/7e92a667e3829831c31e8d46aefe7ef67ad5be1c/gfx/wr/webrender/src/renderer.rs#4292