diff --git a/build.gradle b/build.gradle
index 143e57b76..ba432242b 100644
--- a/build.gradle
+++ b/build.gradle
@@ -1,5 +1,4 @@
 // Top-level build file where you can add configuration options common to all sub-projects/modules.
-
 buildscript {
     apply from: 'versions.gradle'
 
@@ -20,6 +19,14 @@ buildscript {
     }
 }
 
+plugins {
+    id "com.github.spotbugs" version "2.0.0" apply(false)
+}
+
+// Workaround to be able to access SpotBugsTask from external gradle script.
+// More info: https://discuss.gradle.org/t/buildscript-dependencies-in-external-script/23243
+project.extensions.extraProperties.set('SpotBugsTask', com.github.spotbugs.SpotBugsTask)
+
 allprojects {
 
     configurations.all {
diff --git a/config/quality/quality.gradle b/config/quality/quality.gradle
index f7f6f45ec..f76738cbd 100755
--- a/config/quality/quality.gradle
+++ b/config/quality/quality.gradle
@@ -3,7 +3,7 @@
  *
  * Gradle tasks added:
  * - checkstyle
- * - findbugs
+ * - spotbugs
  * - pmd
  *
  * The three tasks above are added as dependencies of the check task so running check will
@@ -11,7 +11,7 @@
  */
 
 apply plugin: 'checkstyle'
-apply plugin: 'findbugs'
+apply plugin: 'com.github.spotbugs'
 apply plugin: 'pmd'
 
 dependencies {
@@ -21,7 +21,7 @@ dependencies {
 def qualityConfigDir = "$project.rootDir/config/quality"
 def reportsDir = "$project.buildDir/reports"
 
-check.dependsOn 'checkstyle', 'findbugs', 'pmd'
+check.dependsOn 'checkstyle', 'spotbugs', 'pmd'
 
 task checkstyle(type: Checkstyle, group: 'Verification', description: 'Runs code style checks') {
     configFile file("$qualityConfigDir/checkstyle/checkstyle-config.xml")
@@ -38,36 +38,37 @@ task checkstyle(type: Checkstyle, group: 'Verification', description: 'Runs code
     classpath = files()
 }
 
-task findbugs(type: FindBugs,
-        group: 'Verification',
-        description: 'Inspect java bytecode for bugs',
-        dependsOn: ['compileDebugSources', 'compileReleaseSources']) {
-
+spotbugs {
     ignoreFailures = false
     effort = "max"
     reportLevel = "high"
-    excludeFilter = new File("$qualityConfigDir/findbugs/android-exclude-filter.xml")
+
+    includeFilter = file("**/*.java")
+    excludeFilter = file("**/gen/**")
+}
+
+tasks.withType(SpotBugsTask) {
+    dependsOn 'assemble'
+    group = "verification"
+    excludeFilter = new File("$qualityConfigDir/spotbugs/android-exclude-filter.xml")
     classes = files("$project.rootDir/folioreader/build/intermediates/javac")
 
-    source 'src'
-    include '**/*.java'
-    exclude '**/gen/**'
+    source = fileTree('src')
 
     reports {
         xml.enabled = false
         html.enabled = true
         xml {
-            destination = file("$reportsDir/findbugs/findbugs.xml")
+            destination = file("$reportsDir/spotbugs/spotbugs.xml")
         }
         html {
-            destination = file("$reportsDir/findbugs/findbugs.html")
+            destination = file("$reportsDir/spotbugs/spotbugs.html")
         }
     }
 
     classpath = files()
 }
 
-
 task pmd(type: Pmd, group: 'Verification', description: 'Inspect sourcecode for bugs') {
     ruleSetFiles = files("$qualityConfigDir/pmd/pmd-ruleset.xml")
     ignoreFailures = false
diff --git a/folioreader/res/layout/folio_activity.xml b/folioreader/res/layout/folio_activity.xml
index b4a01b5ee..fc65623ba 100644
--- a/folioreader/res/layout/folio_activity.xml
+++ b/folioreader/res/layout/folio_activity.xml
@@ -24,7 +24,8 @@
         android:layout_height="wrap_content"
         app:layout_constraintEnd_toEndOf="parent"
         app:layout_constraintStart_toStartOf="parent"
-        app:layout_constraintTop_toTopOf="parent">
+        app:layout_constraintTop_toTopOf="parent"
+        android:animateLayoutChanges="true">
 
         <androidx.appcompat.widget.Toolbar
             android:id="@+id/toolbar"
diff --git a/folioreader/res/layout/progress_dialog.xml b/folioreader/res/layout/progress_dialog.xml
index 0d823fe0e..7f7d5ee62 100644
--- a/folioreader/res/layout/progress_dialog.xml
+++ b/folioreader/res/layout/progress_dialog.xml
@@ -14,7 +14,7 @@
             android:id="@+id/loading"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
-            style="?android:attr/android:progressBarStyle" />
+            style="?android:attr/progressBarStyle" />
 
         <TextView
             android:id="@+id/label_loading"
diff --git a/folioreader/src/main/java/com/folioreader/ui/activity/FolioActivity.kt b/folioreader/src/main/java/com/folioreader/ui/activity/FolioActivity.kt
index 8df26fbe8..2f5c98ae1 100644
--- a/folioreader/src/main/java/com/folioreader/ui/activity/FolioActivity.kt
+++ b/folioreader/src/main/java/com/folioreader/ui/activity/FolioActivity.kt
@@ -696,13 +696,14 @@ class FolioActivity : AppCompatActivity(), FolioActivityCallback, MediaControlle
         distractionFreeMode = visibility != View.SYSTEM_UI_FLAG_VISIBLE
         Log.v(LOG_TAG, "-> distractionFreeMode = $distractionFreeMode")
 
-        if (actionBar != null) {
-            if (distractionFreeMode) {
-                actionBar!!.hide()
-            } else {
-                actionBar!!.show()
-            }
+        if (distractionFreeMode) {
+            appBarLayout?.visibility = View.GONE
+            actionBar?.hide()
+        } else {
+            appBarLayout?.visibility = View.VISIBLE
+            actionBar?.show()
         }
+
     }
 
     override fun toggleSystemUI() {
@@ -717,41 +718,20 @@ class FolioActivity : AppCompatActivity(), FolioActivityCallback, MediaControlle
     private fun showSystemUI() {
         Log.v(LOG_TAG, "-> showSystemUI")
 
-        if (Build.VERSION.SDK_INT >= 16) {
-            val decorView = window.decorView
-            decorView.systemUiVisibility = (View.SYSTEM_UI_FLAG_LAYOUT_STABLE
-                    or View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
-                    or View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN)
-        } else {
-            window.clearFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN)
-            if (appBarLayout != null)
-                appBarLayout!!.setTopMargin(statusBarHeight)
-            onSystemUiVisibilityChange(View.SYSTEM_UI_FLAG_VISIBLE)
-        }
+        window.decorView.systemUiVisibility = (View.SYSTEM_UI_FLAG_LAYOUT_STABLE
+                or View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
+                or View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN)
     }
 
     private fun hideSystemUI() {
         Log.v(LOG_TAG, "-> hideSystemUI")
 
-        if (Build.VERSION.SDK_INT >= 16) {
-            val decorView = window.decorView
-            decorView.systemUiVisibility = (View.SYSTEM_UI_FLAG_IMMERSIVE
-                    // Set the content to appear under the system bars so that the
-                    // content doesn't resize when the system bars hide and show.
-                    or View.SYSTEM_UI_FLAG_LAYOUT_STABLE
-                    or View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
-                    or View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
-                    // Hide the nav bar and status bar
-                    or View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
-                    or View.SYSTEM_UI_FLAG_FULLSCREEN)
-        } else {
-            window.setFlags(
-                WindowManager.LayoutParams.FLAG_FULLSCREEN or WindowManager.LayoutParams.FLAG_LAYOUT_NO_LIMITS,
-                WindowManager.LayoutParams.FLAG_FULLSCREEN or WindowManager.LayoutParams.FLAG_LAYOUT_NO_LIMITS
-            )
-            // Specified 1 just to mock anything other than View.SYSTEM_UI_FLAG_VISIBLE
-            onSystemUiVisibilityChange(1)
-        }
+        window.decorView.systemUiVisibility = (View.SYSTEM_UI_FLAG_LAYOUT_STABLE
+                or View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
+                or View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
+                or View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
+                or View.SYSTEM_UI_FLAG_FULLSCREEN // hide status bar
+                or View.SYSTEM_UI_FLAG_IMMERSIVE)
     }
 
     override fun getEntryReadLocator(): ReadLocator? {
diff --git a/gradle/wrapper/gradle-wrapper.properties b/gradle/wrapper/gradle-wrapper.properties
index 57054cde5..4406e2cbf 100644
--- a/gradle/wrapper/gradle-wrapper.properties
+++ b/gradle/wrapper/gradle-wrapper.properties
@@ -1,6 +1,6 @@
-#Tue Sep 25 12:48:54 IST 2018
+#Wed Jun 24 19:36:22 BRT 2020
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-5.4.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.1.1-all.zip
diff --git a/sample/build.gradle b/sample/build.gradle
index 54caa7c76..965560e4c 100644
--- a/sample/build.gradle
+++ b/sample/build.gradle
@@ -64,11 +64,4 @@ dependencies {
     implementation "com.fasterxml.jackson.core:jackson-databind:$versions.jackson"
     implementation "com.fasterxml.jackson.module:jackson-module-kotlin:$versions.jackson"
 
-    //progressBar error solve
-    configurations.matching { it.name == '_internal_aapt2_binary' }.all { config ->
-        config.resolutionStrategy.eachDependency { details ->
-            details.useVersion("3.3.2-5309881")
-        }
-    }
-
 }
diff --git a/versions.gradle b/versions.gradle
index ec456b157..bf833f022 100644
--- a/versions.gradle
+++ b/versions.gradle
@@ -11,8 +11,8 @@ versions.androidMinSdk = 21
 versions.androidCompileSdk = 28
 versions.androidTargetSdk = 28
 
-versions.androidGradlePlugin = "3.5.3"
-versions.kotlin = "1.3.11"
+versions.androidGradlePlugin = '4.0.0'
+versions.kotlin = "1.3.72"
 
 versions.appcompat = "1.1.0"
 versions.constraintLayout = "1.1.3"