Recently I came across this issue in my gmail a/c. Usually, we have some bookmarks saved in Chrome. Whenever we open a new tab, the bookmarks show up. We can choose the website we want to go to.
When I opened my gmail a/c, for the past 3 days, a new compose box opened up. It was a little annoying. So I googled to find out a solution.
Then I found out that I had a bookmark of " gmail with a new compose box opened."
This was the reason , whenever I chose that bookmark to open my gmail , it automatically opened with a new compose box.
So, I deleted that bookmark for the gmail and opened a new one by typing gmail.com in the browser.
YAY ! I fixed it now.