diff --git a/launcher/BaseInstance.cpp b/launcher/BaseInstance.cpp
index 69cf95e3c..ccfd0b847 100644
--- a/launcher/BaseInstance.cpp
+++ b/launcher/BaseInstance.cpp
@@ -411,3 +411,8 @@ void BaseInstance::updateRuntimeContext()
 {
     // NOOP
 }
+
+bool BaseInstance::isLegacy()
+{
+    return traits().contains("legacyLaunch") || traits().contains("alphaLaunch");
+}
diff --git a/launcher/BaseInstance.h b/launcher/BaseInstance.h
index 2be28d1ec..9827a08b4 100644
--- a/launcher/BaseInstance.h
+++ b/launcher/BaseInstance.h
@@ -269,6 +269,8 @@ class BaseInstance : public QObject, public std::enable_shared_from_this<BaseIns
     bool removeLinkedInstanceId(const QString& id);
     bool isLinkedToInstanceId(const QString& id) const;
 
+    bool isLegacy();
+
    protected:
     void changeStatus(Status newStatus);
 
diff --git a/launcher/minecraft/MinecraftInstance.cpp b/launcher/minecraft/MinecraftInstance.cpp
index 98ac7647f..5be3802bc 100644
--- a/launcher/minecraft/MinecraftInstance.cpp
+++ b/launcher/minecraft/MinecraftInstance.cpp
@@ -91,6 +91,9 @@
 #include "tools/BaseProfiler.h"
 
 #include <QActionGroup>
+#include <QMainWindow>
+#include <QScreen>
+#include <QWindow>
 
 #ifdef Q_OS_LINUX
 #include "MangoHud.h"
@@ -530,7 +533,7 @@ QStringList MinecraftInstance::javaArguments()
 QString MinecraftInstance::getLauncher()
 {
     // use legacy launcher if the traits are set
-    if (traits().contains("legacyLaunch") || traits().contains("alphaLaunch"))
+    if (isLegacy())
         return "legacy";
 
     return "standard";
@@ -752,11 +755,34 @@ QString MinecraftInstance::createLaunchScript(AuthSessionPtr session, MinecraftT
     // window size, title and state, legacy
     {
         QString windowParams;
-        if (settings()->get("LaunchMaximized").toBool())
-            windowParams = "maximized";
-        else
+        if (settings()->get("LaunchMaximized").toBool()) {
+            // FIXME doesn't support maximisation
+            if (!isLegacy()) {
+                auto screen = QGuiApplication::primaryScreen();
+                auto screenGeometry = screen->availableSize();
+
+                // small hack to get the widow decorations
+                for (auto w : QApplication::topLevelWidgets()) {
+                    auto mainWindow = qobject_cast<QMainWindow*>(w);
+                    if (mainWindow) {
+                        auto m = mainWindow->windowHandle()->frameMargins();
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 14, 0))
+                        screenGeometry = screenGeometry.shrunkBy(m);
+#else
+                        screenGeometry = { screenGeometry.width() - m.left() - m.right(), screenGeometry.height() - m.top() - m.bottom() };
+#endif
+                        break;
+                    }
+                }
+
+                windowParams = QString("%1x%2").arg(screenGeometry.width()).arg(screenGeometry.height());
+            } else {
+                windowParams = "maximized";
+            }
+        } else {
             windowParams =
                 QString("%1x%2").arg(settings()->get("MinecraftWinWidth").toInt()).arg(settings()->get("MinecraftWinHeight").toInt());
+        }
         launchScript += "windowTitle " + windowTitle() + "\n";
         launchScript += "windowParams " + windowParams + "\n";
     }
diff --git a/launcher/ui/pages/global/MinecraftPage.ui b/launcher/ui/pages/global/MinecraftPage.ui
index 7d2741250..3a28c92c7 100644
--- a/launcher/ui/pages/global/MinecraftPage.ui
+++ b/launcher/ui/pages/global/MinecraftPage.ui
@@ -55,6 +55,16 @@
             </property>
            </widget>
           </item>
+          <item>
+           <widget class="QLabel" name="maximizedWarning">
+            <property name="toolTip">
+             <string>On newer versions the game only supports resolution. In order to simulate the maximized behaviour the current implementation approximates the maximum display size.</string>
+            </property>
+            <property name="text">
+             <string>&lt;html&gt;&lt;head/&gt;&lt;body&gt;&lt;p&gt;&lt;span style=&quot; font-weight:600; color:#f5c211;&quot;&gt;Warning&lt;/span&gt;&lt;span style=&quot; color:#f5c211;&quot;&gt;: On the newer Minecraft versions the start maximized option is not fully supported.&lt;/span&gt;&lt;/p&gt;&lt;/body&gt;&lt;/html&gt;</string>
+            </property>
+           </widget>
+          </item>
           <item>
            <layout class="QGridLayout" name="gridLayoutWindowSize">
             <item row="1" column="0">
diff --git a/launcher/ui/pages/instance/InstanceSettingsPage.cpp b/launcher/ui/pages/instance/InstanceSettingsPage.cpp
index cf8d86cd4..1a5be050f 100644
--- a/launcher/ui/pages/instance/InstanceSettingsPage.cpp
+++ b/launcher/ui/pages/instance/InstanceSettingsPage.cpp
@@ -323,6 +323,7 @@ void InstanceSettingsPage::loadSettings()
     // Window Size
     ui->windowSizeGroupBox->setChecked(m_settings->get("OverrideWindow").toBool());
     ui->maximizedCheckBox->setChecked(m_settings->get("LaunchMaximized").toBool());
+    ui->maximizedWarning->setVisible(m_settings->get("LaunchMaximized").toBool() && !m_instance->isLegacy());
     ui->windowWidthSpinBox->setValue(m_settings->get("MinecraftWinWidth").toInt());
     ui->windowHeightSpinBox->setValue(m_settings->get("MinecraftWinHeight").toInt());
 
diff --git a/launcher/ui/pages/instance/InstanceSettingsPage.ui b/launcher/ui/pages/instance/InstanceSettingsPage.ui
index 4905eae87..e5ef98b00 100644
--- a/launcher/ui/pages/instance/InstanceSettingsPage.ui
+++ b/launcher/ui/pages/instance/InstanceSettingsPage.ui
@@ -36,7 +36,7 @@
    <item>
     <widget class="QTabWidget" name="settingsTabs">
      <property name="currentIndex">
-      <number>0</number>
+      <number>1</number>
      </property>
      <widget class="QWidget" name="minecraftPage">
       <attribute name="title">
@@ -285,6 +285,16 @@
             </property>
            </widget>
           </item>
+          <item>
+           <widget class="QLabel" name="maximizedWarning">
+            <property name="toolTip">
+             <string>The base game only supports resolution. In order to simulate the maximized behaviour the current implementation approximates the maximum display size..</string>
+            </property>
+            <property name="text">
+             <string>&lt;html&gt;&lt;head/&gt;&lt;body&gt;&lt;p&gt;&lt;span style=&quot; font-weight:600; color:#f5c211;&quot;&gt;Warning&lt;/span&gt;&lt;span style=&quot; color:#f5c211;&quot;&gt;: The maximized option may not be fully supported for the current minecraft version.&lt;/span&gt;&lt;/p&gt;&lt;/body&gt;&lt;/html&gt;</string>
+            </property>
+           </widget>
+          </item>
           <item>
            <layout class="QGridLayout" name="gridLayoutWindowSize">
             <item row="1" column="0">
diff --git a/libraries/launcher/org/prismlauncher/launcher/impl/StandardLauncher.java b/libraries/launcher/org/prismlauncher/launcher/impl/StandardLauncher.java
index dc518be64..084fbc849 100644
--- a/libraries/launcher/org/prismlauncher/launcher/impl/StandardLauncher.java
+++ b/libraries/launcher/org/prismlauncher/launcher/impl/StandardLauncher.java
@@ -76,13 +76,10 @@ public final class StandardLauncher extends AbstractLauncher {
     @Override
     public void launch() throws Throwable {
         // window size, title and state
-        // FIXME doesn't support maximisation
-        if (!maximize) {
-            gameArgs.add("--width");
-            gameArgs.add(Integer.toString(width));
-            gameArgs.add("--height");
-            gameArgs.add(Integer.toString(height));
-        }
+        gameArgs.add("--width");
+        gameArgs.add(Integer.toString(width));
+        gameArgs.add("--height");
+        gameArgs.add(Integer.toString(height));
 
         if (serverAddress != null) {
             if (quickPlayMultiplayerSupported) {