fix(gui): limit the spare memory to max. 512MiB (#434)

jadx-gui/src/main/java/jadx/gui/ui/HeapUsageBar.java

 package jadx.gui.ui;
 
 import jadx.gui.utils.NLS;
+import jadx.gui.utils.Utils;
 
 import javax.swing.*;
 import java.awt.*;
@@ -44,7 +45,7 @@ public class HeapUsageBar extends JProgressBar implements ActionListener {
 		setValue(usedKB);
 		setString(String.format(textFormat, (usedKB / TWO_TO_20), maxGB));
 
-		if (used > r.maxMemory() * 0.8) {
+		if ((used + Utils.MIN_FREE_MEMORY) > r.maxMemory()) {
 			setForeground(RED);
 		} else {
 			setForeground(GREEN);

jadx-gui/src/main/java/jadx/gui/utils/Utils.java

@@ -26,6 +26,16 @@ public class Utils {
 
 	public static final Font FONT_HACK = openFontTTF("Hack-Regular");
 
+	/**
+	 * The minimum about of memory in bytes we are trying to keep free, otherwise the application may run out of heap
+	 * which ends up in a Java garbage collector running "amok" (CPU utilization 100% for each core and the UI is
+	 * not responsive).
+	 *
+	 * We can calculate and store this value here as the maximum heap is fixed for each JVM instance
+	 * and can't be changed at runtime.
+	 */
+	public static final long MIN_FREE_MEMORY = calculateMinFreeMemory();
+
 	private Utils() {
 	}
 
@@ -107,11 +117,21 @@ public class Utils {
 		return overIcon;
 	}
 
+	/**
+	 * @return 20% of the maximum heap size limited to 512 MB (bytes)
+	 */
+	public static long calculateMinFreeMemory() {
+		Runtime runtime = Runtime.getRuntime();
+		long minFree = (long) (runtime.maxMemory() * 0.2);
+		minFree = Math.min(minFree, 512 * 1048576);
+		return minFree;
+	}
+
 	public static boolean isFreeMemoryAvailable() {
 		Runtime runtime = Runtime.getRuntime();
 		long maxMemory = runtime.maxMemory();
-		long totalFree = runtime.freeMemory() + maxMemory - runtime.totalMemory();
-		return totalFree > maxMemory * 0.2;
+		long totalFree = runtime.freeMemory() + (maxMemory - runtime.totalMemory());
+		return totalFree > MIN_FREE_MEMORY;
 	}
 
 	public static String memoryInfo() {