The key part is here: https://searchfox.org/mozilla-central/rev/3fa5cc437a4937c621ea068ba5dc246f75831633/gfx/thebes/gfxPlatformGtk.cpp#456 We need to move mUseWebGLDmabufBackend flag to GetDMABufDevice class which works independently, perhaps as nsDMABufDevice::DisableDMABufWebGL().
Bug 1729656 Comment 8 Edit History
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The key part is here: https://searchfox.org/mozilla-central/rev/3fa5cc437a4937c621ea068ba5dc246f75831633/gfx/thebes/gfxPlatformGtk.cpp#456 We need to move mUseWebGLDmabufBackend flag to GetDMABufDevice class which works independently, perhaps as nsDMABufDevice::DisableDMABufWebGL().