use WebviewPanel (#2781) (#2783)

Author: bobbrow

Extension/src/common.ts

@@ -167,8 +167,16 @@ export function getProgressExecutableSuccess(): number { return progressExecutab
 export function getProgressParseRootSuccess(): number { return progressParseRootSuccess; } // Parse root was successful (i.e. not blocked by processing taking too long).
 export function getProgressIntelliSenseNoSquiggles(): number { return progressIntelliSenseNoSquiggles; } // IntelliSense was successful and the user got no squiggles.
 
-export function showReleaseNotes(): void {
-    vscode.commands.executeCommand('vscode.previewHtml', vscode.Uri.file(getExtensionFilePath("ReleaseNotes.html")), vscode.ViewColumn.One, "C/C++ Extension Release Notes");
+let releaseNotesPanel: vscode.WebviewPanel = undefined;
+
+export async function showReleaseNotes(): Promise<void> {
+    if (releaseNotesPanel) {
+        releaseNotesPanel.reveal();
+    } else {
+        releaseNotesPanel = vscode.window.createWebviewPanel('releaseNotes', "C/C++ Extension Release Notes", vscode.ViewColumn.One);
+        releaseNotesPanel.webview.html = await readFileText(getExtensionFilePath("ReleaseNotes.html"));
+        releaseNotesPanel.onDidDispose(() => releaseNotesPanel = undefined, null, extensionContext.subscriptions);
+    }
 }
 
 export function isUri(input: any): input is vscode.Uri {