Attaching over app.cdp_url points automation at a browser the user is driving,
so two behaviors that are correct for a browser we own are wrong there.
pickElectronTarget enumerated CDP targets and took the first usable one, which
is not necessarily the tab in front of the user, and #captureScreenshot always
called page.bringToFront(), which switches the user's visible tab and pulls
window focus on every screenshot.
Connected browsers now prefer a tab that reports document.visibilityState
"visible" and skip the pre-capture activation, accepting the compositor stall
risk that activation avoids. Headless and spawned browsers are unchanged.
Nik Divjak