The composer has some issues when you have a gifv in the post you’re writing (e.g. in the bug thread I just made). In the preview, the video reloads every time I type something, jumping back up to the top of the preview. This can cause the browser to hang if I type a full sentence fairly quickly or sometimes selecting text.
On Chrome/Win7/Desktop. Can repro while editing the aforementioned thread. Most noticeable when composer height is smaller than gifv height, made writing that post rather difficult. 