fix: avoid window.ipfs in non-HTML context

Injecting proxy into application/xml produced malformed XML and SVG files.
This adds some sanity-checks and ensures injection happens only in HTML
contexts. It also improves performance by avoiding unnecessary port creation.

add-on/src/contentScripts/ipfs-proxy/content.js

@@ -7,14 +7,12 @@ const injectScript = require('./inject-script')
 
 function init () {
   const port = browser.runtime.connect({ name: 'ipfs-proxy' })
-
   // Forward on messages from background to the page and vice versa
   port.onMessage.addListener((data) => {
     if (data && data.sender && data.sender.startsWith('postmsg-rpc/')) {
       window.postMessage(data, '*')
     }
   })
-
   window.addEventListener('message', (msg) => {
     if (msg.data && msg.data.sender && msg.data.sender.startsWith('postmsg-rpc/')) {
       port.postMessage(msg.data)
@@ -24,8 +22,30 @@ function init () {
   injectScript(rawCode)
 }
 
-// Restricting window.ipfs to Secure Context
-// See: https://github.com/ipfs-shipyard/ipfs-companion/issues/476
-if (window.isSecureContext) {
+function injectIpfsProxy () {
+  // Skip if proxy is already present
+  if (window.ipfs) {
+    return false
+  }
+  // Restricting window.ipfs to Secure Context
+  // See: https://github.com/ipfs-shipyard/ipfs-companion/issues/476
+  if (!window.isSecureContext) {
+    return false
+  }
+  // Skip if not in HTML context
+  // Check 1/2
+  const doctype = window.document.doctype
+  if (doctype && doctype.name !== 'html') {
+    return false
+  }
+  // Check 2/2
+  if (document.documentElement.nodeName !== 'HTML') {
+    return false
+  }
+  // Should be ok by now
+  return true
+}
+
+if (injectIpfsProxy()) {
   init()
 }