feat(ui): redetect scale factor on each launch when not set in config (PR260)

* re-detect DPI scale each launch instead of using stale saved value

* make uiScale config default null

---------

Co-authored-by: Jindra Petřík <jindra.petrik@gmail.com>

Author: takattowo

libsrc/ffdec_lib/src/com/jpexs/decompiler/flash/configuration/Configuration.java

@@ -823,7 +823,7 @@ public final class Configuration {
     @ConfigurationCategory("script")
     public static ConfigurationItem<Boolean> flattenASPackages = null;
 
-    @ConfigurationDefaultDouble(1.0)
+    //@ConfigurationDefaultDouble(1.0)
     @ConfigurationCategory("display")
     @ConfigurationName("gui.scale")
     public static ConfigurationItem<Double> uiScale = null;

src/com/jpexs/decompiler/flash/gui/Main.java

@@ -2971,16 +2971,19 @@ private static void initGui() {
         System.setProperty("sun.java2d.d3d", "false");
         System.setProperty("sun.java2d.noddraw", "true");
 
-        if (System.getProperty("sun.java2d.uiScale") == null) { //it was not set by commandline, etc.       
-            if (!Configuration.uiScale.hasValue()) {
+        if (System.getProperty("sun.java2d.uiScale") == null) { //it was not set by commandline, etc.
+            double scaleToUse = Configuration.uiScale.get();
+            if (!Configuration.uiScale.hasValue() || Configuration.uiScale.get() == null) {
+                // Auto-detect from current screen. Do NOT persist to config so the scale is
+                // re-detected on each launch (prevents stale 4K scale being applied on a
+                // lower-DPI monitor after a display change).
                 GraphicsConfiguration configuration = View.getMainDefaultScreenDevice().getDefaultConfiguration();
-
                 AffineTransform transform = configuration.getDefaultTransform();
                 if (transform != null) {
-                    Configuration.uiScale.set(transform.getScaleX());
+                    scaleToUse = transform.getScaleX();
                 }
             }
-            System.setProperty("sun.java2d.uiScale", "" + Configuration.uiScale.get());
+            System.setProperty("sun.java2d.uiScale", "" + scaleToUse);
         }
 
         if (Configuration.hwAcceleratedGraphics.get()) {