On mobile, the "close as duplicate" dialog adapts its size according to a nominated original with all its answers (with quite a lot of content sometimes if the original question is very popular). So it's necessary to scroll all the way down to get to the "Vote" button.
On the desktop, the dialog is fixed-sized and all the stuff is inside this frame.
This is how it looks like (the screenshot are the entire screen without upper-status bar)
I select a Close a duplicate option and see this dialog. Note that it is a) full screen, b) bigger than screen.
I select a question about ternary operator as original. The question with its answers is downloaded into the dialog. Note: there are 28 answers.
Now I have to scroll down all the answers (maybe not all 28 but at least 10 it was, I just didn't count) until I see the Flag button. The scroll indicator is marked with the green arrow.
And then the bottom of the dialog appears.