I really don't wanna sulk or appear smart-ass-ish, don't get me wrong - Geany is nice, but the (missing) window/tabbing flexibility is a big downside for me. Is it that much of a problem to make the windows/tabs more flexible? (Don't know about GLADE, but Qt4 or Wx have that stuff kinda built-in) See Code::Blocks for example - you can drag a tab to one border of the main document frame to create a new document frame on the respective side equal to the first one - without any restrictions on copy/paste or tabbing. Furthermore, dragging off a tab to create a new window (or just the context menu option to detach a tab to create a new window with one document frame and the current tab in it) would be great too.
I just don't see the current "split view" plugin is going anywhere, it's a hackerish add-on without any flexibility which (I guess) leads to all kinds of strange side effects such as the copy/paste issue.
For starters, detatching tabs/moving tabs between 2 windows would be a great workaround (probably with the possibility to hide the documents/symbols- and the status/message/terminal/..-pane automatically in the newly created window cause it's mostly needed just once per open session/project.
Thanks for at least reading this ;)