fix: paintWhenInitiallyHidden on Windows/Linux

shell/browser/api/electron_api_browser_window.cc

@@ -49,6 +49,7 @@ BrowserWindow::BrowserWindow(gin::Arguments* args,
   // when initially hidden
   bool paint_when_initially_hidden = true;
   options.Get(options::kPaintWhenInitiallyHidden, &paint_when_initially_hidden);
+  paint_when_initially_hidden_ = paint_when_initially_hidden;
   if (!paint_when_initially_hidden) {
     bool show = true;
     options.Get(options::kShow, &show);
@@ -288,6 +289,17 @@ void BrowserWindow::OnWindowHide() {
   BaseWindow::OnWindowHide();
 }
 
+void BrowserWindow::RenderViewReady() {
+  // When paintWhenInitiallyHidden is true and the native window has not been
+  // shown yet, tell the WebContents it is visible so the renderer's compositor
+  // starts producing frames.  Without this the renderer's LayerTreeHost stays
+  // hidden and no CompositorFrames (and therefore no presentation callbacks)
+  // are ever produced, which means PerformanceObserver paint-timing entries
+  // (first-paint / first-contentful-paint) never fire.
+  if (paint_when_initially_hidden_ && !window()->IsVisible())
+    web_contents()->WasShown();
+}
+
 void BrowserWindow::Show() {
   web_contents()->WasShown();
   BaseWindow::Show();

shell/browser/api/electron_api_browser_window.h

@@ -42,6 +42,7 @@ class BrowserWindow : public BaseWindow,
 
   // content::WebContentsObserver:
   void BeforeUnloadDialogCancelled() override;
+  void RenderViewReady() override;
   void WebContentsDestroyed() override;
 
   // ExtendedWebContentsObserver:
@@ -79,6 +80,12 @@ class BrowserWindow : public BaseWindow,
  private:
   // Helpers.
 
+  // When true and the window is created with show: false, the renderer is
+  // told it is visible as soon as it is ready so that PerformanceObserver
+  // paint‐timing entries (first-paint / first-contentful-paint) are
+  // produced even before the native window is shown.
+  bool paint_when_initially_hidden_ = false;
+
   v8::Global<v8::Value> web_contents_;
   v8::Global<v8::Value> web_contents_view_;
   base::WeakPtr<api::WebContents> api_web_contents_;

spec/api-browser-window-spec.ts

@@ -6721,28 +6721,35 @@ describe('BrowserWindow module', () => {
     w.loadFile(path.join(fixtures, 'pages', 'send-after-node.html'));
   });
 
-  // TODO(codebytere): fix on Windows and Linux too
-  ifdescribe(process.platform === 'darwin')('window.webContents initial paint', () => {
+  describe('window.webContents initial paint', () => {
     afterEach(closeAllWindows);
-    it('paints when a window is initially hidden', async () => {
-      const w = new BrowserWindow({ show: false });
+    it('paints when a window is initially hidden with paintWhenInitiallyHidden', async () => {
+      const w = new BrowserWindow({
+        show: false,
+        paintWhenInitiallyHidden: true
+      });
       await w.loadFile(path.join(fixtures, 'pages', 'a.html'));
 
       const entries = await w.webContents.executeJavaScript(`
         new Promise((resolve) => {
-          const observer = new PerformanceObserver((performance) => {
-            observer.disconnect();
-            resolve(performance.getEntries());
+          const observer = new PerformanceObserver((list) => {
+            const paintEntries = list.getEntries().filter(
+              e => e.name === 'first-paint' || e.name === 'first-contentful-paint'
+            );
+            if (paintEntries.length > 0) {
+              observer.disconnect();
+              resolve(paintEntries.map(e => e.name));
+            }
           });
           observer.observe({ entryTypes: ['paint'] });
-        });
 
-        const header = document.createElement('h1');
-        header.innerText = 'Paint me!!';
-        document.getElementById('div').appendChild(header);
+          const header = document.createElement('h1');
+          header.innerText = 'Paint me!!';
+          document.getElementById('div').appendChild(header);
+        });
       `);
 
-      expect(JSON.stringify(entries)).to.eq('{}');
+      expect(entries).to.be.an('array').that.includes('first-contentful-paint');
     });
   });