content.ts: Fix pages autofocusing elements

Some pages manage to focus a hidden input field on page load. This puts
tridactyl in insert mode without the user being aware of it. This commit
actively protects users who set `allowautofocus` to "false" against
this.

Closes https://github.com/tridactyl/tridactyl/issues/1492

src/content.ts

@@ -72,6 +72,47 @@ document.addEventListener("readystatechange", _ =>
     getAllDocumentFrames().forEach(f => listen(f)),
 )
 
+// Prevent pages from automatically focusing elements on load
+config.getAsync("allowautofocus").then(allowautofocus => {
+    if (allowautofocus === "true") {
+        return
+    }
+    const preventAutoFocus = () => {
+        // First, blur whatever element is active. This will make sure
+        // activeElement is the "default" active element
+        ; (document.activeElement as any).blur()
+        const elem = document.activeElement as any
+        // ???: We need to set tabIndex, otherwise we won't get focus/blur events!
+        elem.tabIndex = 0
+        const focusElem = () => elem.focus()
+        elem.addEventListener("blur", focusElem)
+        elem.addEventListener("focusout", focusElem)
+        // On top of blur/focusout events, we need to periodically check the
+        // activeElement is the one we want because blur/focusout events aren't
+        // always triggered when document.activeElement changes
+        const interval = setInterval(() => { if (document.activeElement != elem) focusElem() }, 10)
+        // When the user starts interacting with the page, stop resetting focus
+        function stopResettingFocus() {
+            elem.removeEventListener("blur", focusElem)
+            elem.removeEventListener("focusout", focusElem)
+            clearInterval(interval)
+            window.removeEventListener("keydown", stopResettingFocus)
+            window.removeEventListener("mousedown", stopResettingFocus)
+        }
+        window.addEventListener("keydown", stopResettingFocus)
+        window.addEventListener("mousedown", stopResettingFocus)
+    }
+    const tryPreventAutoFocus = () => {
+        document.removeEventListener("readystatechange", tryPreventAutoFocus)
+        try {
+            preventAutoFocus()
+        } catch (e) {
+            document.addEventListener("readystatechange", tryPreventAutoFocus)
+        }
+    }
+    tryPreventAutoFocus()
+})
+
 // Add various useful modules to the window for debugging
 import * as commandline_content from "@src/content/commandline_content"
 import * as convert from "@src/lib/convert"