What I'd like is a trustworthy key-combo (preferably a single key, SysRq seems a good candidate) that toggles an overlay on the screen indicating what applicati

What I'd like is a trustworthy key-combo (preferably a single key, SysRq seems a good candidate) that toggles an overlay on the screen indicating what application "owns" each visible portion, with a very distinctive style identifying parts drawn by the OS. All obscured-input text fields would, by default, only be enabled when the overlay is visible. The overlay would always be available so any app that asks for any sensitive info could use the same mechanism, and I could use it even when the developer didn't think the information was sensitive, or didn't attempt to protect the input channel for any other reason.