Adds buttons to Gmail to quickly toggle the unread filter on the current view. Appears left of the search text input.

filter-gmail-unread.user.js

// ==UserScript==
// @name         Filter Gmail to unread
// @namespace    http://tampermonkey.net/
// @version      1.1.1
// @description  Adds a button to Gmail to quickly toggle the unread filter on the current view.
// @author       jackcarey
// @match        https://mail.google.com/mail/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=google.com
// @grant        none
// @run-at       document-idle
// @homepageURL  https://gist.github.com/jackcarey/b3f65e67b1fe4544096c02d05cc065c6
// @downloadURL  https://gist.github.com/jackcarey/b3f65e67b1fe4544096c02d05cc065c6/raw/filter-gmail-unread.user.js
// @updateURL  https://gist.github.com/jackcarey/b3f65e67b1fe4544096c02d05cc065c6/raw/filter-gmail-unread.user.js
// ==/UserScript==

function waitForElement(selector) {
    return new Promise((resolve) => {
        if (document.querySelector(selector)) {
            return resolve(document.querySelector(selector));
        }

        const observer = new MutationObserver(() => {
            if (document.querySelector(selector)) {
                resolve(document.querySelector(selector));
                observer.disconnect();
            }
        });

        observer.observe(document.querySelector("body"), {
            childList: true,
            subtree: true,
        });
    });
}

waitForElement(`button[aria-label="Search mail"]`).then(function() {
    'use strict';
    const searchInputEl = document.querySelector(`*[name="q"]`);
    const searchBtnEl = document.querySelector(`button[aria-label="Search mail"]`);
    const unreadBtn = document.createElement("button");
    const archivedBtn = document.createElement("button");
    const btnSize = "1.5em";
    const enterEvent = new KeyboardEvent('keydown', {
        key: 'Enter',
        code: 'Enter',
        which: 13,
        keyCode: 13,
    });
    const clickToggler = (querySegment)=>{
        return (evt)=>{
            evt.preventDefault();
            if(!searchInputEl?.value?.includes(querySegment)){
                searchInputEl.value+=` ${querySegment}`;
            }else{
                searchInputEl.value = searchInputEl.value.replace(querySegment,"").trim();
            }
            while(searchInputEl.value.includes("  ")){
                searchInputEl.value = searchInputEl.value.replace("  "," ");
            }
            searchInputEl.value = searchInputEl.value.replace(new RegExp("\s{2,}","gi")," ");
            searchInputEl.value = searchInputEl.value.trim();
            searchInputEl.dispatchEvent(enterEvent);
        };
    };
    //Unread button
    unreadBtn.style.width=btnSize;
    unreadBtn.style.height=btnSize;
    unreadBtn.position="relative";
    unreadBtn.style.top= "1em";
    unreadBtn.style.left= "-2.5em";
    unreadBtn.style.backgroundSize = "contain";
    unreadBtn.title="Toggle unread messages in current view";
    unreadBtn.style.backgroundImage="url(https://ssl.gstatic.com/ui/v1/icons/mail/gm3/1x/stacked_email_baseline_nv700_20dp.png)";
    unreadBtn.className = searchBtnEl.className;
    unreadBtn.addEventListener("click",clickToggler("-is:read"));
    ///Archived button
    archivedBtn.style.width=btnSize;
    archivedBtn.style.height=btnSize;
    archivedBtn.position="relative";
    archivedBtn.style.top= "1em";
    archivedBtn.style.left= "-5em";
    archivedBtn.style.backgroundSize = "contain";
    archivedBtn.title="Toggle archived messages in current view";
    archivedBtn.style.backgroundImage="url(https://ssl.gstatic.com/ui/v1/icons/mail/gm3/1x/inbox_baseline_nv700_20dp.png)";
    archivedBtn.className = searchBtnEl.className;
    archivedBtn.addEventListener("click",clickToggler("in:inbox"));

    //append buttons
    searchBtnEl.after(unreadBtn);
    searchBtnEl.after(archivedBtn);
});

Sounds really interesting. I'll keep monitoring this space. :-)