Cursor disapears on textbox

I have a app that has both a webbrowser control and a textbox. The textbox is used to show the contents of the HTML document and the webbrowser is use to run the HTML

OK, I want to edit the HTML in the textbox so I put the cursor, which shows the blinking cursor, at the point where I want to edit, make changes, save document and run again. If I mouse down anywhere in the browser window the cursor disappears from the textbox. In other words, when I mouse down on the text box there is no cursor visible.