Fix completion on part of existing string

src/OmniSharp.Abstractions/Models/v1/Completion/CompletionItem.cs

@@ -61,10 +61,17 @@ public class CompletionItem
         public string? InsertText { get; set; }
 
         /// <summary>
-        /// The format of <see cref="InsertText"/>.
+        /// The format of <see cref="InsertText"/>. This applies to both <see cref="InsertText"/> and
+        /// <see cref="TextEdit"/>.<see cref="LinePositionSpanTextChange.NewText"/>.
         /// </summary>
         public InsertTextFormat? InsertTextFormat { get; set; }
 
+        /// <summary>
+        /// An edit which is applied to a document when selecting this completion. When an edit is provided the value of
+        /// <see cref="InsertText"/> is ignored.
+        /// </summary>
+        public LinePositionSpanTextChange? TextEdit { get; set; }
+
         /// <summary>
         /// An optional set of characters that when pressed while this completion is active will accept it first and
         /// then type that character.

src/OmniSharp.Roslyn.CSharp/Services/Completion/CompletionService.cs

@@ -162,6 +162,9 @@ public async Task<CompletionResponse> Handle(CompletionRequest request)
             bool expectingImportedItems = expandedItemsAvailable && _workspace.Options.GetOption(CompletionItemExtensions.ShowItemsFromUnimportedNamespaces, LanguageNames.CSharp) == true;
             var syntax = await document.GetSyntaxTreeAsync();
 
+            var replacingSpanStartPosition = sourceText.Lines.GetLinePosition(typedSpan.Start);
+            var replacingSpanEndPosition = sourceText.Lines.GetLinePosition(typedSpan.End);
+
             for (int i = 0; i < completions.Items.Length; i++)
             {
                 var completion = completions.Items[i];
@@ -233,14 +236,30 @@ public async Task<CompletionResponse> Handle(CompletionRequest request)
 
                                 var change = await completionService.GetChangeAsync(document, completion);
 
-                                // If the span we're using to key the completion off is the same as the replacement
-                                // span, then we don't need to do anything special, just snippitize the text and
-                                // exit
                                 if (typedSpan == change.TextChange.Span)
                                 {
+                                    // If the span we're using to key the completion off is the same as the replacement
+                                    // span, then we don't need to do anything special, just snippitize the text and
+                                    // exit
                                     (insertText, insertTextFormat) = getAdjustedInsertTextWithPosition(change, position, newOffset: 0);
                                     break;
                                 }
+                                if (change.TextChange.Span.Start > typedSpan.Start)
+                                {
+                                    // If the span we're using to key the replacement span within the original typed span
+                                    // span, we want to prepend the missing text from the original typed text to here. The
+                                    // reason is that some lsp clients, such as vscode, use the range from the text edit as
+                                    // the selector for what filter text to use. This can lead to odd scenarios where invoking
+                                    // completion and typing `EQ` will bring up the Equals override, but then dismissing and
+                                    // reinvoking completion will have a range that just replaces the Q. Vscode will then consider
+                                    // that capital Q to be the start of the filter text, and filter out the Equals overload
+                                    // leaving the user with no completion and no explanation
+                                    Debug.Assert(change.TextChange.Span.End == typedSpan.End);
+                                    var prefix = typedText.Substring(0, change.TextChange.Span.Start - typedSpan.Start);
+
+                                    (insertText, insertTextFormat) = getAdjustedInsertTextWithPosition(change, position, newOffset: 0, prefix);
+                                    break;
+                                }
 
                                 int additionalEditEndOffset;
                                 (additionalTextEdits, additionalEditEndOffset) = await GetAdditionalTextEdits(change, sourceText, (CSharpParseOptions)syntax!.Options, typedSpan, completion.DisplayText, providerName);
@@ -271,7 +290,14 @@ public async Task<CompletionResponse> Handle(CompletionRequest request)
                 completionsBuilder.Add(new CompletionItem
                 {
                     Label = completion.DisplayTextPrefix + completion.DisplayText + completion.DisplayTextSuffix,
-                    InsertText = insertText,
+                    TextEdit = new LinePositionSpanTextChange
+                    {
+                        NewText = insertText,
+                        StartLine = replacingSpanStartPosition.Line,
+                        StartColumn = replacingSpanStartPosition.Character,
+                        EndLine = replacingSpanEndPosition.Line,
+                        EndColumn = replacingSpanEndPosition.Character
+                    },
                     InsertTextFormat = insertTextFormat,
                     AdditionalTextEdits = additionalTextEdits,
                     // Ensure that unimported items are sorted after things already imported.
@@ -359,7 +385,8 @@ static ImmutableArray<char> buildCommitCharacters(CSharpCompletionList completio
             static (string, InsertTextFormat) getAdjustedInsertTextWithPosition(
                 CompletionChange change,
                 int originalPosition,
-                int newOffset)
+                int newOffset,
+                string? prependText = null)
             {
                 // We often have to trim part of the given change off the front, but we
                 // still want to turn the resulting change into a snippet and control
@@ -375,7 +402,7 @@ static ImmutableArray<char> buildCommitCharacters(CSharpCompletionList completio
                 if (!(change.NewPosition is int newPosition)
                     || newPosition >= (change.TextChange.Span.Start + newText.Length))
                 {
-                    return (newText.Substring(newOffset), InsertTextFormat.PlainText);
+                    return (prependText + newText.Substring(newOffset), InsertTextFormat.PlainText);
                 }
 
                 if (newPosition < (originalPosition + newOffset))
@@ -389,7 +416,7 @@ static ImmutableArray<char> buildCommitCharacters(CSharpCompletionList completio
                 // requested start to the new position, and from the new position to the end of the
                 // string.
                 int midpoint = newPosition - change.TextChange.Span.Start;
-                var beforeText = LspSnippetHelpers.Escape(newText.Substring(newOffset, midpoint - newOffset));
+                var beforeText = LspSnippetHelpers.Escape(prependText + newText.Substring(newOffset, midpoint - newOffset));
                 var afterText = LspSnippetHelpers.Escape(newText.Substring(midpoint));
 
                 return (beforeText + "$0" + afterText, InsertTextFormat.Snippet);