The hard part was understanding what the Flash player was doing, and the differences between all the different Flash Player and SWF File Format versions. Not to mention, re-implementing the rendering engine and the entire API surface of Flash, which was HUGE. We basically took existing content and tried to make sure Shumway did the same thing the Flash player did, wrote a regression test and repeated the process.

To this day, I still don't understand exactly how the timeline worked (or was supposed to work). It's like a web page DOM, but you can jump back and forth in the timeline, it's wild.

We certainly learned a lot working on Shumway. Firefox got a lot better, we stressed the JS engine and the graphics stack quite a bit, and had to make improvements all throughout the codebase to make Shumway possible.

In a lot of ways, Flash was great. Its rendering engine was (and still is) quite impressive, and nothing has really replaced it yet.