fix: modified editor width(microsoft#175397)

Author: yshaojun

src/vs/editor/browser/widget/diffEditorWidget.ts

@@ -1368,7 +1368,7 @@ export class DiffEditorWidget extends Disposable implements editorBrowser.IDiffE
 		this._originalDomNode.style.width = splitPoint + 'px';
 		this._originalDomNode.style.left = '0px';
 
-		this._modifiedDomNode.style.width = (width - splitPoint) + 'px';
+		this._modifiedDomNode.style.width = (width - splitPoint - DiffEditorWidget.ENTIRE_DIFF_OVERVIEW_WIDTH) + 'px';
 		this._modifiedDomNode.style.left = splitPoint + 'px';
 
 		this._overviewDomElement.style.top = '0px';