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/DocumentAdapter.java
private String fContents;
			public void run() {
				if (!isClosed())
					fDocument.set(fContents);
			}
			public void set(String contents) {
				fContents= contents;
				Display.getDefault().syncExec(this);
			}
		}
		/**
		 * Executes a document replace call in the UI thread.
		 */
		protected class DocumentReplaceCommand implements Runnable {
			private int fOffset;
			private int fLength;
			private String fText;
			public void run() {
				try {
					if (!isClosed())
						fDocument.replace(fOffset, fLength, fText);
				} catch (BadLocationException x) {
					
				}
			}
			public void replace(int offset, int length, String text) {
				fOffset= offset;
				fLength= length;
				fText= text;
				Display.getDefault().syncExec(this);