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Code change looks good, just needs more information to work out if this button mapping is consistent for all mice and the same on both X11 and Wayland. See #152128 for more discussion.

See #163503 which interprets the main three buttons more clearly.

I adjusted the description about related issues.

I'm open to adding more information concerning the "magic" numbers used here (8 & 9). It would certainly help to know all the platforms this patch would affect. My personal aim was just for Linux OS desktop environments, but research from @robert-ancell implies there are other implicated platforms here.

@robert-ancell Thank you for the quick responses and deep dives related to this!

The only platform affected here is Linux. If this was a cross-platform GTK app this would be more complicated, but since the Flutter Linux embedder only runs on Linux we don't have to worry about that. The only subplatforms are X11 vs Wayland, but it seems they send the same button value, which is probably expected as they ultimately come from the kernel.

Thanks for all the clarification. I will try to distill the info you've posted into commented context.

I couldn't find any guideline about naming const expressions. I just went with kMouseButtonBack and kMouseButtonForward (based on clang-tidy output). I'm open to suggestions if they need to be more specific (like kLinuxMouseButton***).

I've asked on Discord for a test exemption for this - I looked at writing a test but it was very hard to generate the fake mouse events. In the end the test was just confirming the mapping was the mapping, so I don't think it would add much value anyway.

#164356 is the same change but with restructuring to make it testable.

Excellent work!

Should I close this in favor of #164356 ?