Fix #59210

src/vs/workbench/api/node/extHostOutputService.ts

@@ -94,6 +94,11 @@ export class ExtHostOutputChannelBackedByFile extends AbstractExtHostOutputChann
 		this.validate();
 		this._appender.append(value);
 	}
+
+	clear(): void {
+		this._appender.flush();
+		super.clear();
+	}
 }
 
 export class ExtHostLogFileOutputChannel extends AbstractExtHostOutputChannel {

src/vs/workbench/parts/output/electron-browser/outputServices.ts

@@ -351,12 +351,13 @@ class FileOutputChannel extends AbstractFileOutputChannel implements OutputChann
 	}
 
 	loadModel(): TPromise<ITextModel> {
-		return this.fileService.resolveContent(this.file, { position: this.startOffset, encoding: 'utf8' })
+		return this.readContent()
-			.then(content => {
+			.then(content => this.createModel(content));
-				this.endOffset = this.startOffset + Buffer.from(content.value).byteLength;
+	}
-				this.etag = content.etag;
+
-				return this.createModel(content.value);
+	clear(): void {
-			});
+		this.readContent() // Read content from the file before clearing
+			.then(() => super.clear());
 	}
 
 	append(message: string): void {
@@ -391,6 +392,15 @@ class FileOutputChannel extends AbstractFileOutputChannel implements OutputChann
 		this.updateInProgress = false;
 	}
 
+	private readContent(): TPromise<string> {
+		return this.fileService.resolveContent(this.file, { position: this.startOffset, encoding: 'utf8' })
+			.then(content => {
+				this.endOffset = this.startOffset + Buffer.from(content.value).byteLength;
+				this.etag = content.etag;
+				return content.value;
+			});
+	}
+
 	private onDidContentChange(size: number): void {
 		if (!this.updateInProgress) {
 			this.updateInProgress = true;