My iPhone 15 doesn’t show full screen on my gaming monitor.

It's probable that various factors contribute to this issue, rather than solely Apple's intention to promote their Studio Display. Here's why you might encounter black bars on your screen and some potential solutions:

1. Monitor Aspect Ratio Mismatch: Your gaming monitor likely features a widescreen aspect ratio, whereas your iPhone's display is taller and narrower. Consequently, the image from your phone may not perfectly fit the monitor, resulting in either stretching or black bars.
2. Adapter Limitations: The Apple USB-C Digital AV Multiport Adapter may not be optimized to deliver full-screen output on all monitors, particularly high-resolution gaming monitors. Its design caters more towards general displays rather than specialized gaming setups.

Here are a few troubleshooting steps you can try:

• Adjust Monitor Settings: Explore your gaming monitor's settings for options such as "Just Scan" or "1:1" to display the incoming signal without distortion.
• Try a Different Adapter: Consider using a USB-C to HDMI adapter explicitly designed to support high-resolution or widescreen displays, as it may offer better compatibility.

If you encounter any challenges with third-party applications, the best course of action is to directly contact the app developer for further assistance. Here's a brief guide on how to do it:

1. Identify the app developer.
2. Visit their website or app store listing.
3. Look for their contact information or support channels.
4. Clearly describe the issue you are facing.
5. Provide any necessary information they may need.
6. Follow their instructions and provide feedback.
7. Be patient and follow up if needed.

By reaching out to the app developer, you increase your chances of receiving accurate and personalized support for the challenges you're experiencing. To contact an App developer --> How to contact an app developer - Apple Support