While prototyping a fix for bug 1627073, I discovered that AccGroupInfo::Update is being called a lot more than it should. It turns out that while we do clear the eGroupInfoDirty flag when we already had GroupInfo and then update it, we do not clear eGroupInfoDirty when we first create GroupInfo. Since we just set eGroupInfoDirty without checking whether there is GroupInfo, that means the flag might still be set after GroupInfo is created. Effectively, that means the cache very frequently doesn't get used.

The initial NVDA buffer render for a simple 7000 item <ul> currently takes ~16.4 seconds. If I clear the flag on GroupInfo creation, that drops to ~4.2 seconds. That's a 3.9x perf boost.

When the tree is mutated, we set eGroupInfoDirty regardless of whether there is GroupInfo.
Previously, we cleared eGroupInfoDirty when updating GroupInfo, but not when creating it.
This meant that the flag was often still set, causing the cache to not be used a lot of the time.

https://hg.mozilla.org/mozilla-central/rev/6998e2f1e2d8