Cache container position for DomUtil.getMousePosition

[mourner]

Profiler says that most of the time during mouse interactions is spent on getting the current mouse position relative to map container, and specifically container position (container.getBoundingClientRect() and container.clientLeft/Top).

We could cache it under certain conditions to get rid of the slowdown, but this is somewhat tricky because we need to avoid out-of-sync situations. One way is to put a window.onscroll handler when we mouseover the container, reset cache on scroll and use cached otherwise. But this may have side effects so needs to be tested thoroughly.

[mourner]

cc @danzel

[mourner]

Weird thing about that is that getBoundingClientRect only takes a lot of time on heavy examples such as drawing lots of features with Canvas, but it's not related to DOM tree size since Canvas is one element... Mystery.

[danzel]

Could you share an example this is easy to reproduce on?
What you've suggested sounds like a good plan if the hit is meaningful :)

[danzel]

Are we manipulating the DOM, then calling those functions?
If we are then we might be causing a reflow

[mourner]

It's easy to reproduce with the debug/vector/geojson.html example — just start the profiler and mouseover like crazy over states. I've made the optimization above (cache-mouse-pos branch), but for some weird reason, while it shows a significant improvement in profiler, it seems not to affect real-world performance at all (e.g. FF lags on bug geojson as much as it was, while Chrome is as smooth with the patch and without). So closing for now.