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The part of the desktop on the second display, where the taskbar would be (if it weren't on the primary display), is unavailable. No windows can be moved to this area. It is as if the taskbar was there, like on the primary display.
A workaround is to set the Hide Mode to "Allow other windows to cover the panel" (in Configure Panel > Hiding), but I don't want this setting on the primary display. My setting is "Only hide when a panel-hiding button is clicked".
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Update ---
This problem only happens when the taskbar panel arrangement is set to the Top edge. When it is set to the Bottom edge (default position), it works fine.
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