Sometimes it is browser related.

You can click outside the editor window and use your up/down arrows to move the invisible part of the screen back into view.

Check your "view" or "zoom", it may be set at 150% or something when you need something smaller.  

I have had this happen to me only occasionally. FINALLY, after some experimentation, I can get this to happen!!! Now we can report it to AR devs so they can work on the solution. 
The key to AVOID this problem is to NOT DRAG ON THE SCROLL BAR to go down the page. If you do, the page only renders about halfway down and the bottom half of the page appears blank.

To scroll down, use one of these options instead:
* roll the wheel on the mouse to go up or down the page
* use two fingers on the touchpad and slide up or down to scroll
* use the page down/page up keys (or the up and down arrow keys)

If you get stuck where the bottom half of the page is blank. Just drag the scroll bar back UP ALL THE WAY TO THE TOP. Afterward, scroll down on the page NOT with the scroll bar but one of the methods listed above.

cc: ActiveRain Member Support 

try a different browser

Mark,

The post is within a box and you just scroll up and down.  I use Safari as a browser, and also Firefox.  Both work well.  And I think that ActiveRain Member Support gave you the best explanation. A

It is frustrating for sure.  I use Chrome and it will sometimes happen to me as well.  Sometimes scrolling the box does not work effectively either.

When that happens I save the post (word doc on my hard drive as well as in AR) log out and log back in again. That will most of the time correct the issue. 

This is a CSS issue.

Developers:
I found if you add position:relative; to #body-section #main-section #content-holder.sections-2, it works.

You have:

#body-section #main-section #content-holder.sections-2 {
    width: 62%;
    overflow: hidden;
    margin-right: 1%;
}

change it to:

#body-section #main-section #content-holder.sections-2 {
    width: 62%;
    overflow: hidden;
    margin-right: 1%;
    position:relative;

}

It worked for me every time a tried it.

By the way, I am using a chrome extension (Live CSS Editor) to add CSS on the fly.

I haven't seen that (yet) but I'm glad that you are able to experiment and inform AR.  Hopefully this is going to be resolved soon.

ActiveRain Memmber Support to the rescue. Others have had this issue!

I use Firefox and occasionally have this happen. When I refresh it seems fine... HOWEVER, be sure to save anything you're working on first!

I have heard others say this too, but have not experienced this.   I would change browsers, or like Silvia mentioned, check the size shown on the screen.

Try using Chrome.

For me, it only happens with I'm using Chrome. When I use Safari, it literally NEVER happens! I was talking to someone in another line of business and they have a similar issue with Chrome. 

Interesting problem

It's definitely a problem and I have it occasionally.  I usually monkey around with the scroll going up and down until the whole page populates.  It's not a consistent problem.

Only happens in Chrome & I am used to the workaround already. Also clear your cache as that seems to help also.

Try the Firefox browser.