Add fallback Html panel support for markdown rendering (#522)

Android Studio frustratingly ships with a broken version of the embedded
markdown plugin, which prevents use of the modern JCEF API we were
using. This adds a fallback implementation for when that's not available
and just uses a simpler html panel. Code blocks don't work and it's a
little plain, but it's otherwise ok.

<img width="1512" alt="Screenshot 2023-08-10 at 12 56 05 AM"
src="https://github.com/slackhq/slack-gradle-plugin/assets/1361086/6df182be-dcdd-49c8-b01c-12a1052c1b08">


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skate-plugin/src/main/kotlin/com/slack/sgp/intellij/ui/WhatsNewPanelFactory.kt

@@ -18,6 +18,7 @@ package com.slack.sgp.intellij.ui
 import com.intellij.openapi.Disposable
 import com.intellij.openapi.application.runReadAction
 import com.intellij.openapi.components.service
+import com.intellij.openapi.diagnostic.logger
 import com.intellij.openapi.project.DumbAware
 import com.intellij.openapi.project.Project
 import com.intellij.openapi.util.Disposer
@@ -26,6 +27,12 @@ import com.intellij.testFramework.LightVirtualFile
 import com.intellij.ui.content.ContentFactory
 import com.intellij.ui.content.ContentManagerEvent
 import com.intellij.ui.content.ContentManagerListener
+import com.intellij.ui.dsl.builder.panel
+import com.intellij.ui.dsl.gridLayout.HorizontalAlign
+import com.intellij.ui.dsl.gridLayout.VerticalAlign
+import com.intellij.ui.jcef.JBCefApp
+import com.intellij.util.ui.HtmlPanel
+import com.intellij.util.ui.JBUI
 import com.slack.sgp.intellij.ChangelogJournal
 import com.slack.sgp.intellij.ChangelogParser
 import java.awt.BorderLayout
@@ -71,6 +78,8 @@ class WhatsNewPanelFactory : DumbAware {
     parentDisposable: Disposable
   ) {
 
+    private val logger = logger<WhatsNewPanelContent>()
+
     // Actual panel box for "What's New in Slack!"
     val contentPanel: JPanel =
       JPanel().apply {
@@ -91,15 +100,49 @@ class WhatsNewPanelFactory : DumbAware {
 
       val file = LightVirtualFile("changelog.md", changeLogContent.changeLogString ?: "")
 
-      val panel = MarkdownJCEFHtmlPanel(project, file)
-      Disposer.register(parentDisposable, panel)
-
       val html = runReadAction {
         MarkdownUtil.generateMarkdownHtml(file, changeLogContent.changeLogString ?: "", project)
       }
 
-      panel.setHtml(html, 0)
-      return panel.component
+      // We have to support two different modes here: the modern JCEF-based one and the legacy
+      // Legacy is because Android Studio ships with a broken markdown plugin that doesn't work with
+      // JCEF
+      // https://issuetracker.google.com/issues/159933628#comment19
+      val panel =
+        if (JBCefApp.isSupported()) {
+          logger.debug("Using JCEFHtmlPanelProvider")
+          MarkdownJCEFHtmlPanel(project, file)
+            .apply {
+              Disposer.register(parentDisposable, this)
+              setHtml(html, 0)
+            }
+            .component
+        } else {
+          logger.debug("Using HtmlPanel")
+          val htmlPanel =
+            object : HtmlPanel() {
+              init {
+                isVisible = true
+                // Padding
+                border = JBUI.Borders.empty(16)
+                update()
+              }
+
+              override fun getBody(): String {
+                return html
+              }
+            }
+          panel {
+            row {
+                cell(htmlPanel)
+                  .horizontalAlign(HorizontalAlign.FILL)
+                  .verticalAlign(VerticalAlign.FILL)
+              }
+              .resizableRow()
+          }
+        }
+
+      return panel
     }
   }
 }