Fix back button to never go to a different website using history len check

app/javascript/mastodon/components/column_back_button.js

@@ -9,8 +9,12 @@ export default class ColumnBackButton extends React.PureComponent {
   };
 
   handleClick = () => {
-    if (window.history && window.history.length === 1) this.context.router.history.push('/');
-    else this.context.router.history.goBack();
+    // if history is exhausted, or we would leave mastodon, just go to root.
+    if (window.history && (window.history.length === 1 || window.history.length === window._mastoInitialHistoryLen)) {
+      this.context.router.history.push('/');
+    } else {
+      this.context.router.history.goBack();
+    }
   }
 
   render () {

app/javascript/mastodon/components/column_back_button_slim.js

@@ -9,8 +9,12 @@ export default class ColumnBackButtonSlim extends React.PureComponent {
   };
 
   handleClick = () => {
-    if (window.history && window.history.length === 1) this.context.router.history.push('/');
-    else this.context.router.history.goBack();
+    // if history is exhausted, or we would leave mastodon, just go to root.
+    if (window.history && (window.history.length === 1 || window.history.length === window._mastoInitialHistoryLen)) {
+      this.context.router.history.push('/');
+    } else {
+      this.context.router.history.goBack();
+    }
   }
 
   render () {

app/javascript/mastodon/components/column_header.js

@@ -45,8 +45,12 @@ export default class ColumnHeader extends React.PureComponent {
   }
 
   handleBackClick = () => {
-    if (window.history && window.history.length === 1) this.context.router.history.push('/');
-    else this.context.router.history.goBack();
+    // if history is exhausted, or we would leave mastodon, just go to root.
+    if (window.history && (window.history.length === 1 || window.history.length === window._mastoInitialHistoryLen)) {
+      this.context.router.history.push('/');
+    } else {
+      this.context.router.history.goBack();
+    }
   }
 
   handleTransitionEnd = () => {

app/javascript/mastodon/main.js

@@ -26,6 +26,11 @@ function main() {
 
     ReactDOM.render(<Mastodon {...props} />, mountNode);
     perf.stop('main()');
+
+    // remember the initial URL
+    if (window.history && typeof window._mastoInitialHistoryLen === 'undefined') {
+      window._mastoInitialHistoryLen = window.history.length;
+    }
   });
 }