I have [a circuit simulator that redraws as you drag things around](http://algorithmicassertions.com/quirk) on an HTML5 canvas.

I've noticed that, sometimes, it fails to redraw during drags. It holds the last drawn frame until I stop moving the mouse, then starts showing updated content.

At first I thought this might be some kind of issue with the code I use the throttle the draws below 60fps. Maybe timer events were being swamped out by higher priority UI events or something like that. But I profiled the code in Chrome, and the profiler confirms the draw code is being called and finishing in a reasonable amount of time.

Here's a screenshot from an example profile I collected while dragging with the issue happening. Note the draw code finishing within 5ms, with plenty of idle time:

[![Flame chart](https://i.stack.imgur.com/e540c.png)](https://i.stack.imgur.com/e540c.png)

So mostly I'm stumped and need ideas of what to check.

What are some common reasons for redraws not to show when they are being triggered by user actions?