Some projects keep surprising me with their “solutions,” and this is one of those cases. A proposal under review by developers from GNOME and Mozilla could change how middle-mouse-button paste behaves on Linux and other Unix-like systems.
The discussions, visible in Mozilla’s Phabricator revision D277804 and a linked GNOME gsettings-desktop-schemas merge request, focus on disabling the traditional primary selection paste by default.
Mozilla proposes changing the default behavior of the Firefox browser on Unix builds so that pressing the middle mouse button no longer pastes text by default. The author of the revision frames the current behavior as a source of confusion and accidental pastes, especially when users press the middle button without expecting the clipboard contents to be inserted into text fields.
As I explained elsewhere there is no official app to change this setting. Users can hack their gsettings.
Support for middle-paste will slowly but surely bitrot and eventually be removed.
They explicitly mention the plan to add a toggle in gnome settings in the merge request.
You’re skipping a step here, first a decision needs to be made on whether or not the default will change, then and only then can they decide whether it’s worth adding something like a toggle to the mouse settings panel, which would be trivial btw.
Configuration UI can be added regardless of what happens with defaults. Defaults change is not a blocker for exposing configurability. If anything I’d say you got it backwards: Don’t switch long-standing defaults until there is a discoverable and accessible way to change it.