Adapt line number to be 1-based from a 0-based position.

packages/editor/src/browser/editor-linenumber-contribution.ts

@@ -15,7 +15,7 @@
 // *****************************************************************************
 
 import { EditorManager } from './editor-manager';
-import { EditorMouseEvent, MouseTargetType, TextEditor } from './editor';
+import { EditorMouseEvent, MouseTargetType, Position, TextEditor } from './editor';
 import { injectable, inject, optional } from '@theia/core/shared/inversify';
 import { FrontendApplicationContribution, QuickInputService, ApplicationShell, ContextMenuRenderer } from '@theia/core/lib/browser';
 import { ContextKeyService } from '@theia/core/lib/browser/context-key-service';
@@ -62,10 +62,11 @@ export class EditorLineNumberContribution implements FrontendApplicationContribu
         if (event.target && event.target.type === MouseTargetType.GUTTER_LINE_NUMBERS) {
             if (event.event.button === 2) {
                 editor.focus();
-                const contextKeyService = this.contextKeyService.createOverlay([['editorLineNumber', event.target.position?.line]]);
+                const lineNumber = lineNumberFromPosition(event.target.position);
+                const contextKeyService = this.contextKeyService.createOverlay([['editorLineNumber', lineNumber]]);
                 const uri = editor.getResourceUri()!;
                 const args = [{
-                    lineNumber: event.target.position?.line!,
+                    lineNumber: lineNumber,
                     uri: uri['codeUri']
                 }];
 
@@ -83,3 +84,12 @@ export class EditorLineNumberContribution implements FrontendApplicationContribu
     }
 
 }
+
+function lineNumberFromPosition(position: Position | undefined): number | undefined {
+    // position.line is 0-based line position, where the expected editor line number is 1-based.
+    if (position) {
+        return position.line + 1;
+    }
+    return undefined;
+}
+