This is from Josef Pa from the FF team:
The issue you're experiencing is due to the custom code import of html_editor_enhanced, which depends on flutter_inappwebview. To make it work on the web, you need to make some adjustments. Specifically, you need to add a script tag, as described in the installation instructions here: flutter_inappwebview installation guide.Please add the script attached below to the custom headers within web deployment and it should resolve the issue for you.
The script:
<script type=\"application/javascript\" defer>\nwindow.flutter_inappwebview = {\n    webViews: {},\n    createFlutterInAppWebView: function(viewId, iframeId) {\n        var webView = {\n            viewId: viewId,\n            iframeId: iframeId,\n            iframe: null,\n            iframeContainer: null,\n            windowAutoincrementId: 0,\n            windows: {},\n            isFullscreen: false,\n            documentTitle: null,\n            functionMap: {},\n            settings: {},\n            disableContextMenuHandler: function(event) {\n                event.preventDefault();\n                event.stopPropagation();\n                return false;\n            },\n            prepare: function(settings) {\n                webView.settings = settings;\n                var iframe = document.getElementById(iframeId);\n                var iframeContainer = document.getElementById(iframeId + '-container');\n\n                document.addEventListener('fullscreenchange', function(event) {\n                    // document.fullscreenElement will point to the element that\n                    // is in fullscreen mode if there is one. If there isn't one,\n                    // the value of the property is null.\n                    if (document.fullscreenElement && document.fullscreenElement.id == iframeId) {\n                        webView.isFullscreen = true;\n                        window.flutter_inappwebview.nativeCommunication('onEnterFullscreen', viewId);\n                    } else if (!document.fullscreenElement && webView.isFullscreen) {\n                        webView.isFullscreen = false;\n                        window.flutter_inappwebview.nativeCommunication('onExitFullscreen', viewId);\n                    } else {\n                        webView.isFullscreen = false;\n                    }\n                });\n\n                if (iframe != null) {\n                    webView.iframe = iframe;\n                    webView.iframeContainer = iframeContainer;\n                    iframe.addEventListener('load', function (event) {\n                        webView.windowAutoincrementId = 0;\n                        webView.windows = {};\n\n                        var url = iframe.src;\n                        try {\n                            url = iframe.contentWindow.location.href;\n                        } catch (e) {\n                            console.log(e);\n                        }\n                        window.flutter_inappwebview.nativeCommunication('onLoadStart', viewId, [url]);\n\n                        try {\n                            var oldLogs = {\n                                'log': iframe.contentWindow.console.log,\n                                'debug': iframe.contentWindow.console.debug,\n                                'error': iframe.contentWindow.console.error,\n                                'info': iframe.contentWindow.console.info,\n                                'warn': iframe.contentWindow.console.warn\n                            };\n                            for (var k in oldLogs) {\n                                (function(oldLog) {\n                                    iframe.contentWindow.console[oldLog] = function() {\n                                        var message = '';\n                                        for (var i in arguments) {\n                                            if (message == '') {\n                                                message += arguments[i];\n                                            } else {\n                                                message += ' ' + arguments[i];\n                                            }\n                                        }\n                                        oldLogs[oldLog].call(iframe.contentWindow.console, ...arguments);\n                                        window.flutter_inappwebview.nativeCommunication('onConsoleMessage', viewId, [oldLog, message]);\n                                    }\n                                })(k);\n                            }\n                        } catch (e) {\n                            console.log(e);\n                        }\n\n                        try {\n                            var originalPushState = iframe.contentWindow.history.pushState;\n                            iframe.contentWindow.history.pushState = function (state, unused, url) {\n                                originalPushState.call(iframe.contentWindow.history, state, unused, url);\n                                var iframeUrl = iframe.src;\n                                try {\n                                    iframeUrl = iframe.contentWindow.location.href;\n                                } catch (e) {\n                                    console.log(e);\n                                }\n                                window.flutter_inappwebview.nativeCommunication('onUpdateVisitedHistory', viewId, [iframeUrl]);\n                            };\n\n                            var originalReplaceState = iframe.contentWindow.history.replaceState;\n                            iframe.contentWindow.history.replaceState = function (state, unused, url) {\n                                originalReplaceState.call(iframe.contentWindow.history, state, unused, url);\n                                var iframeUrl = iframe.src;\n                                try {\n                                    iframeUrl = iframe.contentWindow.location.href;\n                                } catch (e) {\n                                    console.log(e);\n                                }\n                                window.flutter_inappwebview.nativeCommunication('onUpdateVisitedHistory', viewId, [iframeUrl]);\n                            };\n\n                            var originalOpen = iframe.contentWindow.open;\n                            iframe.contentWindow.open = function (url, target, windowFeatures) {\n                                var newWindow = originalOpen.call(iframe.contentWindow, ...arguments);\n                                var windowId = webView.windowAutoincrementId;\n                                webView.windowAutoincrementId++;\n                                webView.windows[windowId] = newWindow;\n                                window.flutter_inappwebview.nativeCommunication('onCreateWindow', viewId, [windowId, url, target, windowFeatures]).then(function(){}, function(handledByClient) {\n                                    if (handledByClient) {\n                                        newWindow.close();\n                                    }\n                                });\n                                return newWindow;\n                            };\n\n                            var originalPrint = iframe.contentWindow.print;\n                            iframe.contentWindow.print = function() {\n                                var iframeUrl = iframe.src;\n                                try {\n                                    iframeUrl = iframe.contentWindow.location.href;\n                                } catch (e) {\n                                    console.log(e);\n                                }\n                                window.flutter_inappwebview.nativeCommunication('onPrintRequest', viewId, [iframeUrl]);\n                                originalPrint.call(iframe.contentWindow);\n                            };\n\n                            webView.functionMap = {\n                                \"window.open\": iframe.contentWindow.open,\n                                \"window.print\": iframe.contentWindow.print,\n                                \"window.history.pushState\": iframe.contentWindow.history.pushState,\n                                \"window.history.replaceState\": iframe.contentWindow.history.replaceState,\n                            }\n\n                            var initialTitle = iframe.contentDocument.title;\n                            var titleEl = iframe.contentDocument.querySelector('title');\n                            webView.documentTitle = initialTitle;\n                            window.flutter_inappwebview.nativeCommunication('onTitleChanged', viewId, [initialTitle]);\n                            if (titleEl != null) {\n                                new MutationObserver(function(mutations) {\n                                    var title = mutations[0].target.innerText;\n                                    if (title != webView.documentTitle) {\n                                        webView.documentTitle = title;\n                                        window.flutter_inappwebview.nativeCommunication('onTitleChanged', viewId, [title]);\n                                    }\n                                }).observe(\n                                    titleEl,\n                                    { subtree: true, characterData: true, childList: true }\n                                );\n                            }\n\n                            var oldPixelRatio = iframe.contentWindow.devicePixelRatio;\n                            iframe.contentWindow.addEventListener('resize', function (e) {\n                                var newPixelRatio = iframe.contentWindow.devicePixelRatio;\n                                if(newPixelRatio !== oldPixelRatio){\n                                    window.flutter_inappwebview.nativeCommunication('onZoomScaleChanged', viewId, [oldPixelRatio, newPixelRatio]);\n                                    oldPixelRatio = newPixelRatio;\n                                }\n                            });\n\n                            iframe.contentWindow.addEventListener('popstate', function (event) {\n                                var iframeUrl = iframe.src;\n                                try {\n                                    iframeUrl = iframe.contentWindow.location.href;\n                                } catch (e) {\n                                    console.log(e);\n                                }\n                                window.flutter_inappwebview.nativeCommunication('onUpdateVisitedHistory', viewId, [iframeUrl]);\n                            });\n\n                            iframe.contentWindow.addEventListener('scroll', function (event) {\n                                var x = 0;\n                                var y = 0;\n                                try {\n                                    x = iframe.contentWindow.scrollX;\n                                    y = iframe.contentWindow.scrollY;\n                                } catch (e) {\n                                    console.log(e);\n                                }\n                                window.flutter_inappwebview.nativeCommunication('onScrollChanged', viewId, [x, y]);\n                            });\n\n                            iframe.contentWindow.addEventListener('focus', function (event) {\n                                window.flutter_inappwebview.nativeCommunication('onWindowFocus', viewId);\n                            });\n\n                            iframe.contentWindow.addEventListener('blur', function (event) {\n                                window.flutter_inappwebview.nativeCommunication('onWindowBlur', viewId);\n                            });\n                        } catch (e) {\n                            console.log(e);\n                        }\n\n                        try {\n\n                            if (!webView.settings.javaScriptCanOpenWindowsAutomatically) {\n                                iframe.contentWindow.open = function() {\n                                    throw new Error('JavaScript cannot open windows automatically');\n                                };\n                            }\n\n                            if (!webView.settings.verticalScrollBarEnabled && !webView.settings.horizontalScrollBarEnabled) {\n                                var style = iframe.contentDocument.createElement('style');\n                                style.id = \"settings.verticalScrollBarEnabled-settings.horizontalScrollBarEnabled\";\n                                style.innerHTML = \"body::-webkit-scrollbar { width: 0px; height: 0px; }\";\n                                iframe.contentDocument.head.append(style);\n                            }\n\n                            if (webView.settings.disableVerticalScroll) {\n                                var style = iframe.contentDocument.createElement('style');\n                                style.id = \"settings.disableVerticalScroll\";\n                                style.innerHTML = \"body { overflow-y: hidden; }\";\n                                iframe.contentDocument.head.append(style);\n                            }\n\n                            if (webView.settings.disableHorizontalScroll) {\n                                var style = iframe.contentDocument.createElement('style');\n                                style.id = \"settings.disableHorizontalScroll\";\n                                style.innerHTML = \"body { overflow-x: hidden; }\";\n                                iframe.contentDocument.head.append(style);\n                            }\n\n                            if (webView.settings.disableContextMenu) {\n                                iframe.contentWindow.addEventListener('contextmenu', webView.disableContextMenuHandler);\n                            }\n                        } catch (e) {\n                            console.log(e);\n                        }\n\n                        window.flutter_inappwebview.nativeCommunication('onLoadStop', viewId, [url]);\n                    });\n                }\n            },\n            setSettings: function(newSettings) {\n                var iframe = webView.iframe;\n                try {\n                    if (webView.settings.javaScriptCanOpenWindowsAutomatically != newSettings.javaScriptCanOpenWindowsAutomatically) {\n                        if (!newSettings.javaScriptCanOpenWindowsAutomatically) {\n                            iframe.contentWindow.open = function() {\n                                throw new Error('JavaScript cannot open windows automatically');\n                            };\n                        } else {\n                            iframe.contentWindow.open = webView.functionMap[\"window.open\"];\n                        }\n                    }\n\n                    if (webView.settings.verticalScrollBarEnabled != newSettings.verticalScrollBarEnabled &&\n                        webView.settings.horizontalScrollBarEnabled != newSettings.horizontalScrollBarEnabled) {\n                        if (!newSettings.verticalScrollBarEnabled && !newSettings.horizontalScrollBarEnabled) {\n                            var style = iframe.contentDocument.createElement('style');\n                            style.id = \"settings.verticalScrollBarEnabled-settings.horizontalScrollBarEnabled\";\n                            style.innerHTML = \"body::-webkit-scrollbar { width: 0px; height: 0px; }\";\n                            iframe.contentDocument.head.append(style);\n                        } else {\n                            var styleElement = iframe.contentDocument.getElementById(\"settings.verticalScrollBarEnabled-settings.horizontalScrollBarEnabled\");\n                            if (styleElement) { styleElement.remove() }\n                        }\n                    }\n\n                    if (webView.settings.disableVerticalScroll != newSettings.disableVerticalScroll) {\n                        if (newSettings.disableVerticalScroll) {\n                            var style = iframe.contentDocument.createElement('style');\n                            style.id = \"settings.disableVerticalScroll\";\n                            style.innerHTML = \"body { overflow-y: hidden; }\";\n                            iframe.contentDocument.head.append(style);\n                        } else {\n                             var styleElement = iframe.contentDocument.getElementById(\"settings.disableVerticalScroll\");\n                            if (styleElement) { styleElement.remove() }\n                        }\n                    }\n\n                    if (webView.settings.disableHorizontalScroll != newSettings.disableHorizontalScroll) {\n                        if (newSettings.disableHorizontalScroll) {\n                            var style = iframe.contentDocument.createElement('style');\n                            style.id = \"settings.disableHorizontalScroll\";\n                            style.innerHTML = \"body { overflow-x: hidden; }\";\n                            iframe.contentDocument.head.append(style);\n                        } else {\n                             var styleElement = iframe.contentDocument.getElementById(\"settings.disableHorizontalScroll\");\n                            if (styleElement) { styleElement.remove() }\n                        }\n                    }\n\n                    if (webView.settings.disableContextMenu != newSettings.disableContextMenu) {\n                        if (newSettings.disableContextMenu) {\n                            iframe.contentWindow.addEventListener('contextmenu', webView.disableContextMenuHandler);\n                        } else {\n                            iframe.contentWindow.removeEventListener('contextmenu', webView.disableContextMenuHandler);\n                        }\n                    }\n                } catch (e) {\n                    console.log(e);\n                }\n\n                webView.settings = newSettings;\n            },\n            reload: function() {\n                var iframe = webView.iframe;\n                if (iframe != null && iframe.contentWindow != null) {\n                    try {\n                        iframe.contentWindow.location.reload();\n                    } catch (e) {\n                        console.log(e);\n                        iframe.contentWindow.location.href = iframe.src;\n                    }\n                }\n            },\n            goBack: function() {\n                var iframe = webView.iframe;\n                if (iframe != null) {\n                    try {\n                        iframe.contentWindow.history.back();\n                    } catch (e) {\n                        console.log(e);\n                    }\n                }\n            },\n            goForward: function() {\n                var iframe = webView.iframe;\n                if (iframe != null) {\n                    try {\n                        iframe.contentWindow.history.forward();\n                    } catch (e) {\n                        console.log(e);\n                    }\n                }\n            },\n            goBackOrForward: function(steps) {\n                var iframe = webView.iframe;\n                if (iframe != null) {\n                    try {\n                        iframe.contentWindow.history.go(steps);\n                    } catch (e) {\n                        console.log(e);\n                    }\n                }\n            },\n            evaluateJavascript: function(source) {\n                var iframe = webView.iframe;\n                var result = null;\n                if (iframe != null) {\n                    try {\n                        result = JSON.stringify(iframe.contentWindow.eval(source));\n                    } catch (e) {}\n                }\n                return result;\n            },\n            stopLoading: function(steps) {\n                var iframe = webView.iframe;\n                if (iframe != null) {\n                    try {\n                        iframe.contentWindow.stop();\n                    } catch (e) {\n                        console.log(e);\n                    }\n                }\n            },\n            getUrl: function() {\n                var iframe = webView.iframe;\n                var url = iframe.src;\n                try {\n                    url = iframe.contentWindow.location.href;\n                } catch (e) {\n                    console.log(e);\n                }\n                return url;\n            },\n            getTitle: function() {\n                var iframe = webView.iframe;\n                var title = null;\n                try {\n                    title = iframe.contentDocument.title;\n                } catch (e) {\n                    console.log(e);\n                }\n                return title;\n            },\n            injectJavascriptFileFromUrl: function(urlFile, scriptHtmlTagAttributes) {\n                var iframe = webView.iframe;\n                try {\n                    var d = iframe.contentDocument;\n                    var script = d.createElement('script');\n                    for (var key of Object.keys(scriptHtmlTagAttributes)) {\n                        if (scriptHtmlTagAttributes[key] != null) {\n                            script[key] = scriptHtmlTagAttributes[key];\n                        }\n                    }\n                    if (script.id != null) {\n                        script.onload = function() {\n                            window.flutter_inappwebview.nativeCommunication('onInjectedScriptLoaded', webView.viewId, [script.id]);\n                        }\n                        script.onerror = function() {\n                            window.flutter_inappwebview.nativeCommunication('onInjectedScriptError', webView.viewId, [script.id]);\n                        }\n                    }\n                    script.src = urlFile;\n                    if (d.body != null) {\n                        d.body.appendChild(script);\n                    }\n                } catch (e) {\n                    console.log(e);\n                }\n            },\n            injectCSSCode: function(source) {\n                var iframe = webView.iframe;\n                try {\n                    var d = iframe.contentDocument;\n                    var style = d.createElement('style');\n                    style.innerHTML = source;\n                    if (d.head != null) {\n                        d.head.appendChild(style);\n                    }\n                } catch (e) {\n                    console.log(e);\n                }\n            },\n            injectCSSFileFromUrl: function(urlFile, cssLinkHtmlTagAttributes) {\n                var iframe = webView.iframe;\n                try {\n                    var d = iframe.contentDocument;\n                    var link = d.createElement('link');\n                    for (var key of Object.keys(cssLinkHtmlTagAttributes)) {\n                        if (cssLinkHtmlTagAttributes[key] != null) {\n                            link[key] = cssLinkHtmlTagAttributes[key];\n                        }\n                    }\n                    link.type = 'text/css';\n                    var alternateStylesheet = \"\";\n                    if (cssLinkHtmlTagAttributes.alternateStylesheet) {\n                        alternateStylesheet = \"alternate \";\n                    }\n                    link.rel = alternateStylesheet + \"stylesheet\";\n                    link.href = urlFile;\n                    if (d.head != null) {\n                        d.head.appendChild(link);\n                    }\n                } catch (e) {\n                    console.log(e);\n                }\n            },\n            scrollTo: function(x, y, animated) {\n                var iframe = webView.iframe;\n                try {\n                    if (animated) {\n                        iframe.contentWindow.scrollTo({top: y, left: x, behavior: 'smooth'});\n                    } else {\n                        iframe.contentWindow.scrollTo(x, y);\n                    }\n                } catch (e) {\n                    console.log(e);\n                }\n            },\n            scrollBy: function(x, y, animated) {\n                var iframe = webView.iframe;\n                try {\n                    if (animated) {\n                        iframe.contentWindow.scrollBy({top: y, left: x, behavior: 'smooth'});\n                    } else {\n                        iframe.contentWindow.scrollBy(x, y);\n                    }\n                } catch (e) {\n                    console.log(e);\n                }\n            },\n            printCurrentPage: function() {\n                var iframe = webView.iframe;\n                try {\n                    iframe.contentWindow.print();\n                } catch (e) {\n                    console.log(e);\n                }\n            },\n            getContentHeight: function() {\n                var iframe = webView.iframe;\n                try {\n                    return iframe.contentDocument.documentElement.scrollHeight;\n                } catch (e) {\n                    console.log(e);\n                }\n                return null;\n            },\n            getContentWidth: function() {\n                var iframe = webView.iframe;\n                try {\n                    return iframe.contentDocument.documentElement.scrollWidth;\n                } catch (e) {\n                    console.log(e);\n                }\n                return null;\n            },\n            getSelectedText: function() {\n                var iframe = webView.iframe;\n                try {\n                    var txt;\n                    var w = iframe.contentWindow;\n                    if (w.getSelection) {\n                        txt = w.getSelection().toString();\n                    } else if (w.document.getSelection) {\n                        txt = w.document.getSelection().toString();\n                    } else if (w.document.selection) {\n                        txt = w.document.selection.createRange().text;\n                    }\n                    return txt;\n                } catch (e) {\n                    console.log(e);\n                }\n                return null;\n            },\n            getScrollX: function() {\n                var iframe = webView.iframe;\n                try {\n                    return iframe.contentWindow.scrollX;\n                } catch (e) {\n                    console.log(e);\n                }\n                return null;\n            },\n            getScrollY: function() {\n                var iframe = webView.iframe;\n                try {\n                    return iframe.contentWindow.scrollY;\n                } catch (e) {\n                    console.log(e);\n                }\n                return null;\n            },\n            isSecureContext: function() {\n                var iframe = webView.iframe;\n                try {\n                    return iframe.contentWindow.isSecureContext;\n                } catch (e) {\n                    console.log(e);\n                }\n                return false;\n            },\n            canScrollVertically: function() {\n                var iframe = webView.iframe;\n                try {\n                    return iframe.contentDocument.body.scrollHeight > iframe.contentWindow.innerHeight;\n                } catch (e) {\n                    console.log(e);\n                }\n                return false;\n            },\n            canScrollHorizontally: function() {\n                var iframe = webView.iframe;\n                try {\n                    return iframe.contentDocument.body.scrollWidth > iframe.contentWindow.innerWidth;\n                } catch (e) {\n                    console.log(e);\n                }\n                return false;\n            },\n            getSize: function() {\n                var iframeContainer = webView.iframeContainer;\n                var width = 0.0;\n                var height = 0.0;\n                if (iframeContainer.style.width != null && iframeContainer.style.width != '' && iframeContainer.style.width.indexOf('px') > 0) {\n                    width = parseFloat(iframeContainer.style.width);\n                }\n                if (width == null || width == 0.0) {\n                    width = iframeContainer.getBoundingClientRect().width;\n                }\n                if (iframeContainer.style.height != null && iframeContainer.style.height != '' && iframeContainer.style.height.indexOf('px') > 0) {\n                    height = parseFloat(iframeContainer.style.height);\n                }\n                if (height == null || height == 0.0) {\n                    height = iframeContainer.getBoundingClientRect().height;\n                }\n\n                return {\n                    width: width,\n                    height: height\n                };\n            }\n        };\n\n        return webView;\n    },\n    getCookieExpirationDate: function(timestamp) {\n        return (new Date(timestamp)).toUTCString();\n    }\n};\n</script>
Flutter Version 3.22.2 Upgrade html_editor_enhanced: ^2.6.0 not working
