Fixed-position element not visible on page load in iOS WebView

I’m experiencing an issue with fixed-position elements when a webpage is opened inside WhatsApp’s in-app browser on iOS (which uses Apple’s WebView).

As shown in the attached video, when the page is first loaded, the bottom fixed button (sticky bar) is not visible — it appears as if it’s off-screen or clipped out of view. Once I manually scroll or slightly “wiggle” near the top navigation bar, the fixed element suddenly becomes visible and behaves as expected.

This issue only occurs in the WhatsApp in-app browser. When opening the same link directly in Safari, the fixed button is displayed correctly from the start.

Expected behavior:

The fixed-position bar should be visible immediately on page load, just like it is in Safari.

Environment:

• iOS version: 26
• Device: iPhone 14
• Browser: WhatsApp in-app browser (Apple WebView)

Question:

Is there any known CSS or viewport workaround to ensure fixed-position elements render correctly on page load within iOS WebViews (like the one used in WhatsApp)?