Actually, there has to be a an OK button to complete the task. This is still happening using the PC version of Zotero. I found the same problem trying to use "Document Preferences" in Word 2016 in Windows 7 with the Zotero plugin. In one attempt, the bottom of the window with the "OK" "Cancel" and "Help" buttons was cut off. Then on another attempt, the top of the window was cut off, but the "OK" "Cancel" and "Help" buttons were showing.
Maybe I'm missing what you mean by "online Zotero", but on the website (which is the only online version of Zotero that exists), there is no "operation" to complete and there is definitely not supposed to be an OK button there. It simply a window that displays the generated citation(s).
But if this is happening on Windows from Standalone (like document preferences), that obviously shouldn't be the case. Can you make the window bigger by dragging at the bottom/top? Do you have a particularly small or low-resolution screen?
I found a solution. In the "Display" panel there's an option to "Make text and other items larger or smaller." I had it set at 125%. When I reduced it to the default (100%) the Zotero windows were no longer cut off.
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I found the same problem trying to use "Document Preferences" in Word 2016 in Windows 7 with the Zotero plugin.
In one attempt, the bottom of the window with the "OK" "Cancel" and "Help" buttons was cut off. Then on another attempt, the top of the window was cut off, but the "OK" "Cancel" and "Help" buttons were showing.
But if this is happening on Windows from Standalone (like document preferences), that obviously shouldn't be the case. Can you make the window bigger by dragging at the bottom/top? Do you have a particularly small or low-resolution screen?