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
ProblemRequestorState state= (ProblemRequestorState) fProblemRequestorState.get();
				if (state != null && !state.fInsideReportingSequence)
					internalEndReporting(state);
			}
			/*
			 * @see org.eclipse.jdt.internal.ui.text.java.IProblemRequestorExtension#endReportingSequence()
			 */
			public void endReportingSequence() {
				ProblemRequestorState state= (ProblemRequestorState) fProblemRequestorState.get();
				if (state != null && state.fInsideReportingSequence)
					internalEndReporting(state);
			}
			private void internalEndReporting(ProblemRequestorState state) {
				int stateCount= 0;
				synchronized(getLockObject()) {
					-- fStateCount;
					stateCount= fStateCount;
					fProblemRequestorState.set(null);
				}
				if (stateCount == 0 && fIsHandlingTemporaryProblems)
					reportProblems(state.fReportedProblems);
			}
			/**
			 * Signals the end of problem reporting.
			 */
			private void reportProblems(List reportedProblems) {
				if (fProgressMonitor != null && fProgressMonitor.isCanceled())
					return;
				boolean temporaryProblemsChanged= false;
				synchronized (getLockObject()) {