Windows draws the yellow border, not Fluxer. It shows it around a window or screen while an app captures it through Windows Graphics Capture, the capture feature built into Windows, so that you can see that a capture is in progress.
When does the Fluxer desktop app use Windows' capture feature?
When you share a window from the "Applications" tab, the desktop app captures it with Windows Graphics Capture. When you share your whole screen from the "Entire screen" tab, the desktop app uses Windows Graphics Capture on Windows 11 version 24H2 and later, and a different capture method without a border on older versions of Windows.
Can I turn off the yellow border?
On Windows 10, no. Windows only lets apps capture without the border from Windows build 20348, and no version of Windows 10 is that new, including the latest one.
On Windows 11, apps can ask Windows to capture without the border, but the desktop app has no setting for it. Windows can also show the border whenever another app that captures the same window asks for one, as Microsoft's documentation explains.
How can I share without the yellow border?
On Windows 10, and on Windows 11 before version 24H2, share your whole screen from the "Entire screen" tab, which doesn't use Windows Graphics Capture there. You can also capture a game in OBS Studio and share its virtual camera in Fluxer.