I'm using Chrome. I've been having an issue for the past few weeks--as I'm typing in a forum-thread entry box (like this one), the cursor disappears, and I'm suddenly out of the entry box. So I could be rapidly typing a paragraph, and suddenly my keystrokes aren't being registered. It seems to happen on pages with several inserted ads, and when the page updates (ads, other things?) and I see the activity reported in the lower left corner of the browser window.
It can happen so often, it can be frustrating just to finish one sentence. It requires grabbing the mouse and clicking in the box to get a cursor-entry bar again to continue typing.
I've tested this on a couple of Windows PCs--same issue on all of them.