Viewport cannot be selected by clicking

[fds]

On an existing file, I suddenly cannot select a viewport by just clicking. I need to do Ctrl-A. However, I do not have this problem when I tested it on a brand new file. The relevant layers are on. I can easily create the viewport but cannot just select it. What should I check on?

[Dipali]

if the viewport is drawn on deffpoint layer & layer ' 0 ' is switched off or locked you will not be able to select viewport

[Tiger]

And sometimes even if Layer 0 is on, if the Viewport is on Layer Defpoints, the Viewport still can't be selected - I don't know why, but it has happened to me quite a few times.

[fds]

Thanks Dipali. It worked on 1 file but on other files it didn't seem to work, just like what Tiger explained. Weird, it's the first time I am encounterting this.