Implement (faulty) detection for when to hide/show the button

Seems now blocked on the navigation.canGoBack API being implemented, so waiting on https://bugzilla.mozilla.org/show_bug.cgi?id=1777171

src/duplicate-tab.js

@@ -182,10 +182,33 @@ export default class DuplicateTab {
                 return true
             }
             if (data.getPageData === true) {
+                let oldTabHasHistory = false
+                try {
+                    let injectionResult = (await browser.scripting.executeScript({
+                        target: {
+                            tabId: id
+                        },
+                        // The same code from the file as a function doesn't
+                        // seem able to return the last expression and is always
+                        // undefined, so we need to use a file for this even
+                        // though we just need to execute a single line
+                        files: [ 'src/get-history.js' ]
+                    }))[0]
+                    // Length is 1 when history is empty due to current tab
+                    // FIXME: This doesn't work properly if there are pages
+                    // forward of the current page, as they are included
+                    // in the history too even when we have nothing to go back
+                    // to. `navigation.canGoBack` seems what we actually want
+                    // but it has not been implemented at the time of writing
+                    oldTabHasHistory = injectionResult.result > 1
+                } catch (error) {
+                    console.error('Error checking for history in original tab', error)
+                }
                 return {
                     url: url,
                     oldTabIsIncognito: incognito,
-                    allowedIncognitoAccess: await browser.extension.isAllowedIncognitoAccess()
+                    allowedIncognitoAccess: await browser.extension.isAllowedIncognitoAccess(),
+                    oldTabHasHistory: oldTabHasHistory,
                 }
             }
         } catch (error) {

src/get-history.js

@@ -0,0 +1 @@
+window.history.length

src/page/main.js

@@ -47,8 +47,7 @@ buttons.forEach((b) => b.deleter = new IdempotentElementDeleter(b.element))
         type: 'page',
         getPageData: true
     })
-    const { url } = response
-    const { oldTabIsIncognito } = response
+    const { url, oldTabIsIncognito, oldTabHasHistory } = response
 
     document.getElementById('page-url').textContent += (url)
     document.getElementById('page-url-suffix').classList.remove('hidden')
@@ -89,6 +88,10 @@ buttons.forEach((b) => b.deleter = new IdempotentElementDeleter(b.element))
         let navigateBackTabNav = document.querySelector('#normal-and-navigate-back-tab-name')
         navigateBackTabNav.textContent = 'Private Browsing'
     }
+    if (!oldTabHasHistory) {
+        // nothing to navigate back with so delete the option
+        buttons.get('normal-and-navigate-back').deleter.run()
+    }
     const { allowedIncognitoAccess } = response
     if (allowedIncognitoAccess === false) {
         // the only case that needs user notification is moving