test: wait until the page shows a screen, not only until it lays out (closes #502)

The browser suites' wait for a screen passed as soon as the element laid
out. Until the page's stylesheet has applied, every view lays out, so an
approval test could read the prompt before the page's script had filled
it. visible() in the Chrome harness and waitVisible() in the Firefox
driver now also wait for the page to finish loading and for neither the
element nor anything around it to carry the hidden class that showView()
keeps on every view but the current one. Both also require the element's
computed visibility to be visible, as the browser library's own check in
Chrome did. No caller changed.

Model: opus-5-5

tests/e2e/firefox/driver.js

@@ -290,6 +290,7 @@ class Driver {
             `const el = document.querySelector(arguments[0]);
              if (document.readyState !== "complete" || !el) return false;
              if (el.closest(".hidden")) return false;
+             if (getComputedStyle(el).visibility !== "visible") return false;
              const r = el.getBoundingClientRect();
              return r.width > 0 && r.height > 0;`,
             [selector],

tests/e2e/harness.js

@@ -348,6 +348,7 @@ async function visible(page, selector, timeout = 15000) {
                 const el = document.querySelector(sel);
                 if (document.readyState !== "complete" || !el) return false;
                 if (el.closest(".hidden")) return false;
+                if (getComputedStyle(el).visibility !== "visible") return false;
                 const r = el.getBoundingClientRect();
                 return r.width > 0 && r.height > 0;
             },