Bug 96917 [implementation] If two editors use the 2.x-compatible API, one or both doesn't work
At least one problem is that 3.0 installs partitioners for every editor that might possibly edit a file, but each editor expects its own partitioner to be installed. Leading to the famous white screen with blue line.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/CompilationUnitDocumentProvider.java
public ReverseMap() {
			}
			public Object get(Position position) {
				Entry entry;
				
				int length= fList.size();
				for (int i= fAnchor; i < length; i++) {
					entry= (Entry) fList.get(i);
					if (entry.fPosition.equals(position)) {
						fAnchor= i;
						return entry.fValue;
					}
				}
				
				for (int i= 0; i < fAnchor; i++) {
					entry= (Entry) fList.get(i);
					if (entry.fPosition.equals(position)) {
						fAnchor= i;
						return entry.fValue;
					}
				}
				return null;
			}
			private int getIndex(Position position) {
				Entry entry;
				int length= fList.size();
				for (int i= 0; i < length; i++) {
					entry= (Entry) fList.get(i);
					if (entry.fPosition.equals(position))
						return i;