fix: gate native window reveal on renderer load

Keep BrowserWindow hidden until both ready-to-show and did-finish-load are complete, preventing the incomplete header from reaching the screen.

Render a static upload-speed baseline beneath the transparent canvas so the zero line exists independently of renderer timers.

Add regression coverage for both reveal event orders, load failures, and the script-independent header baseline.

lib/startup-renderer.js

@@ -15,14 +15,31 @@ function createStartupQuery(config, version) {
 
 function createStartupWindow(BrowserWindow, options) {
   const window = new BrowserWindow({ ...options, show: false });
-  window.once('ready-to-show', () => {
+  let nativePaintReady = false;
+  let rendererLoadFinished = false;
+  let revealed = false;
+  const reveal = () => {
+    if (revealed || !nativePaintReady || !rendererLoadFinished) return;
+    revealed = true;
     window.show();
+  };
+  window.once('ready-to-show', () => {
+    nativePaintReady = true;
+    reveal();
+  });
+  window.webContents.once('did-finish-load', () => {
+    rendererLoadFinished = true;
+    reveal();
   });
 
   return {
     window,
     load(target, onLoadError, options) {
-      return window.loadFile(target, options).catch(onLoadError);
+      return window.loadFile(target, options).catch((error) => {
+        rendererLoadFinished = true;
+        reveal();
+        return onLoadError(error);
+      });
     }
   };
 }

renderer/index.html

@@ -55,7 +55,10 @@
     <div class="header-spacer" aria-hidden="true"></div>
     <div class="header-cluster header-utilities">
       <div class="upload-speed-sparkline" id="uploadSpeedSparkline" title="Aktuelle Upload-Geschwindigkeit" aria-label="Aktuelle Upload-Geschwindigkeit">
-        <canvas id="uploadSpeedCanvas" width="232" height="44" aria-hidden="true"></canvas>
+        <span class="upload-speed-chart" aria-hidden="true">
+          <span class="upload-speed-baseline"></span>
+          <canvas id="uploadSpeedCanvas" width="232" height="44"></canvas>
+        </span>
         <strong id="uploadSpeedValue">0 B/s</strong>
       </div>
       <button class="header-update-button" id="headerUpdateBtn" title="Nach Aktualisierungen suchen" aria-label="Nach Aktualisierungen suchen" data-tooltip="Nach Aktualisierungen suchen">

renderer/styles.css

@@ -1960,7 +1960,27 @@ select.hs-input { max-width: none; width: auto; min-width: 140px; }
   transition: opacity .16s ease;
 }
 
+.upload-speed-chart {
+  position: relative;
+  min-width: 0;
+  height: 22px;
+  display: block;
+}
+
+.upload-speed-baseline {
+  position: absolute;
+  left: 0;
+  right: 0;
+  bottom: 2px;
+  height: 2px;
+  border-radius: 999px;
+  background: var(--success);
+  pointer-events: none;
+}
+
 .upload-speed-sparkline canvas {
+  position: relative;
+  z-index: 1;
   width: 100%;
   height: 22px;
   display: block;
@@ -4053,7 +4073,7 @@ input[type="checkbox"] {
     justify-content: flex-end;
   }
 
-  .upload-speed-sparkline canvas {
+  .upload-speed-chart {
     display: none;
   }
 

tests/startup-renderer.test.js

@@ -8,6 +8,7 @@ const { configureStartupRenderer, createStartupWindow, resolveStartupLanguage, c
 class TestBrowserWindow extends EventEmitter {
   constructor(options) {
     super();
+    this.webContents = new EventEmitter();
     this.options = options;
     this.showCalls = 0;
     this.startupEvents = [];
@@ -27,7 +28,7 @@ class TestBrowserWindow extends EventEmitter {
   loadFile(target, options) {
     this.startupEvents.push(`load:${target}`);
     this.loadOptions = options;
-    return Promise.reject(this.loadError);
+    return this.loadError ? Promise.reject(this.loadError) : Promise.resolve();
   }
 }
 
@@ -78,6 +79,7 @@ test('main window uses the branded application icon', () => {
 
 test('startup load registers visibility before navigation and shows only once', async () => {
   const startup = createStartupWindow(TestBrowserWindow, {});
+  startup.window.loadError = null;
   const loading = startup.load('renderer/index.html', () => {});
 
   assert.deepEqual(startup.window.startupEvents, [
@@ -86,12 +88,24 @@ test('startup load registers visibility before navigation and shows only once',
   ]);
 
   startup.window.emit('ready-to-show');
+  assert.equal(startup.window.showCalls, 0);
+  startup.window.webContents.emit('did-finish-load');
   startup.window.emit('ready-to-show');
+  startup.window.webContents.emit('did-finish-load');
   await loading;
 
   assert.equal(startup.window.showCalls, 1);
 });
 
+test('startup waits for native paint readiness when renderer loading finishes first', () => {
+  const startup = createStartupWindow(TestBrowserWindow, {});
+
+  startup.window.webContents.emit('did-finish-load');
+  assert.equal(startup.window.showCalls, 0);
+  startup.window.emit('ready-to-show');
+  assert.equal(startup.window.showCalls, 1);
+});
+
 test('startup load forwards a rejected navigation to the error handler', async () => {
   const startup = createStartupWindow(TestBrowserWindow, {});
   let handledError;
@@ -150,4 +164,6 @@ test('header occupies its final geometry before asynchronous initialization', ()
   assert.ok(firstFrameInitialization < asynchronousInitialization);
   assert.match(mainSource, /createStartupQuery\([^,]+,\s*app\.getVersion\(\)\)/u);
   assert.doesNotMatch(mainSource, /runAutomaticUpdateCheck\(true\);\s*\},\s*3000\)/u);
+  assert.match(html, /class="upload-speed-baseline"/u);
+  assert.match(css, /\.upload-speed-baseline\s*\{[^}]*background:\s*var\(--success\);/su);
 });