Chromium does workaround of scrolling with touch when child window is used for DirectComposition. It looks better to import it to gecko for WebRender.
https://bugs.chromium.org/p/chromium/issues/detail?id=871257

There were 2 bugs that might be affected. Bug 1487564 and Bug 1455518

When I just added WS_EX_LAYERED | WS_EX_TRANSPARENT | WS_EX_NOREDIRECTIONBITMAP, compositor widow creation was just failed :(

(In reply to Sotaro Ikeda [:sotaro] from comment #2)

When I just added WS_EX_LAYERED | WS_EX_TRANSPARENT | WS_EX_NOREDIRECTIONBITMAP, compositor widow creation was just failed :(

When it happened, I set security.sandbox.gpu.level=1 pref by mistake.

With attachment 9041402 [details], talos tresize opt e10s stylo on windows10-64-qr was improved 8.81% though try was unstable today.

https://treeherder.mozilla.org/perf.html#/compare?originalProject=try&originalRevision=74300343702689f2db05ad1ec8ab90ab39dd702d&newProject=try&newRevision=742bcc4fe2dd4babac7bf2623717d040726666cc&framework=1

talos result with today's try. Today try was stable and talos tresize opt e10s stylo on windows10-64-qr was improved 11%.

https://treeherder.mozilla.org/perf.html#/compare?originalProject=try&originalRevision=0f89d4b8b58cace5fdc7bf6da3cc1ed4fac8db76&newProject=try&newRevision=53db67569f04bc0ead5ed82036ae32f1b76c297c&framework=1

https://hg.mozilla.org/mozilla-central/rev/b7efc3a909be