Bug 553830: [R-Editor] Add support for negative replacementOffset
in assist SourceProposal

Change-Id: I85f56944230a841bdc010bdeb7b76a65d35e1b33
diff --git a/r/org.eclipse.statet.r.ui/src/org/eclipse/statet/internal/r/ui/editors/RElementCompletionProposal.java b/r/org.eclipse.statet.r.ui/src/org/eclipse/statet/internal/r/ui/editors/RElementCompletionProposal.java
index 9ba6a40..11d87cc 100644
--- a/r/org.eclipse.statet.r.ui/src/org/eclipse/statet/internal/r/ui/editors/RElementCompletionProposal.java
+++ b/r/org.eclipse.statet.r.ui/src/org/eclipse/statet/internal/r/ui/editors/RElementCompletionProposal.java
@@ -259,7 +259,7 @@
 	@Override
 	protected @Nullable String getValidationPrefix(final int offset) throws BadLocationException {
 		// keep in synch with RSimpleCompletionProposal
-		final int startOffset= getReplacementOffset();
+		final int startOffset= Math.max(getReplacementOffset(), 0);
 		if (offset >= startOffset) {
 			final RAssistInvocationContext context= getInvocationContext();
 			final IDocument document= context.getDocument();
diff --git a/r/org.eclipse.statet.r.ui/src/org/eclipse/statet/internal/r/ui/editors/RSimpleCompletionProposal.java b/r/org.eclipse.statet.r.ui/src/org/eclipse/statet/internal/r/ui/editors/RSimpleCompletionProposal.java
index d9e83a8..bcaa5ce 100644
--- a/r/org.eclipse.statet.r.ui/src/org/eclipse/statet/internal/r/ui/editors/RSimpleCompletionProposal.java
+++ b/r/org.eclipse.statet.r.ui/src/org/eclipse/statet/internal/r/ui/editors/RSimpleCompletionProposal.java
@@ -58,7 +58,7 @@
 	@Override
 	protected @Nullable String getValidationPrefix(final int offset) throws BadLocationException {
 		// keep in synch with RElementCompletionProposal
-		final int startOffset= getReplacementOffset();
+		final int startOffset= Math.max(getReplacementOffset(), 0);
 		if (offset >= startOffset) {
 			final RAssistInvocationContext context= getInvocationContext();
 			final IDocument document= context.getDocument();