LayerManagerComposite does full rendering for taking a snapshot. https://searchfox.org/mozilla-central/source/gfx/layers/composite/LayerManagerComposite.cpp#598
Bug 1593383 Comment 1 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
LayerManagerComposite does full rendering for taking a snapshot. https://searchfox.org/mozilla-central/source/gfx/layers/composite/LayerManagerComposite.cpp#598 LayerManagerMLGPU also does full rendering for taking a snapshot. https://searchfox.org/mozilla-central/source/gfx/layers/mlgpu/LayerManagerMLGPU.cpp#511