CachedImageIcon.scale(value) should be immutable

- it should return new icon instance for value!=1 and it's allowed to return "this" for value==1

platform/platform-tests/testData/ui/abstractClass.svg

+<svg id="Layer_1" data-name="Layer 1" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16">
+  <title>abstractClass</title>
+  <g>
+    <path d="M11,1.68261a6.93358,6.93358,0,0,0-6,0V14.31737a6.93358,6.93358,0,0,0,6,0V1.68261Z" fill="#40b6e0" opacity="0.5" style="isolation: isolate"/>
+    <path d="M10.616,9.98144l-0.5407-.55337a2.47758,2.47758,0,0,1-1.88411.78046A2.12355,2.12355,0,0,1,6.16382,8V7.98339A2.145,2.145,0,0,1,8.22873,5.76118l0.0204-.00065a2.6174,2.6174,0,0,1,1.82321.71283l0.51116-.55467A3.13348,3.13348,0,0,0,8.2804,5H8.09859A3.02986,3.02986,0,0,0,5.00064,7.96019L5,8V8.01635A2.994,2.994,0,0,0,8.00238,11H8.256A3.12367,3.12367,0,0,0,10.616,9.98144Z" fill="#231f20" opacity="0.6" style="isolation: isolate"/>
+    <path d="M4,2.26A6.99125,6.99125,0,0,0,4,13.74V2.26Z" fill="#9aa7b0" opacity="0.6" style="isolation: isolate"/>
+    <path d="M12,2.26V13.74A6.99125,6.99125,0,0,0,12,2.26Z" fill="#9aa7b0" opacity="0.6" style="isolation: isolate"/>
+  </g>
+</svg>

platform/platform-tests/testData/ui/abstractClass@2x.svg

+<svg id="Layer_1" data-name="Layer 1" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <title>abstractClass@2x</title>
+  <g>
+    <path d="M22,3.36522a13.86717,13.86717,0,0,0-12,0V28.63475a13.86717,13.86717,0,0,0,12,0V3.36522Z" fill="#40b6e0" opacity="0.5" style="isolation: isolate"/>
+    <path d="M21.23186,19.96312l-1.0814-1.10674a4.95515,4.95515,0,0,1-3.76821,1.56092,4.2471,4.2471,0,0,1-4.05469-4.41708V15.967a4.28989,4.28989,0,0,1,4.12959-4.44441l0.041-.00131A5.2348,5.2348,0,0,1,20.14461,12.947l1.02231-1.10934a6.267,6.267,0,0,0-4.6062-1.8374H16.1971a6.05971,6.05971,0,0,0-6.19589,5.9204l-0.00129.0796V16.033a5.9881,5.9881,0,0,0,6.00476,5.96729h0.50721A6.24735,6.24735,0,0,0,21.23186,19.96312Z" fill="#231f20" opacity="0.6" style="isolation: isolate"/>
+    <path d="M8,4.52A13.9825,13.9825,0,0,0,8,27.48V4.52Z" fill="#9aa7b0" opacity="0.6" style="isolation: isolate"/>
+    <path d="M24,4.52V27.48A13.9825,13.9825,0,0,0,24,4.52Z" fill="#9aa7b0" opacity="0.6" style="isolation: isolate"/>
+  </g>
+</svg>

platform/platform-tests/testSrc/com/intellij/util/ui/IconScaleTest.java

+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
+package com.intellij.util.ui;
+
+import com.intellij.openapi.util.IconLoader.CachedImageIcon;
+import com.intellij.openapi.util.SystemInfo;
+import com.intellij.openapi.util.registry.Registry;
+import com.intellij.openapi.util.registry.RegistryValue;
+import com.intellij.testFramework.PlatformTestUtil;
+import com.intellij.util.IconUtil;
+import com.intellij.util.ui.JBUI.ScaleContext;
+import com.intellij.util.ui.paint.AbstractPainter2DTest;
+import com.intellij.util.ui.paint.PaintUtilTest;
+import junit.framework.TestCase;
+import org.junit.After;
+import org.junit.Assume;
+import org.junit.Before;
+import org.junit.Test;
+
+import javax.swing.*;
+import java.io.File;
+import java.net.MalformedURLException;
+
+import static com.intellij.util.ui.JBUI.ScaleType.SYS_SCALE;
+
+/**
+ * Tests that {@link CachedImageIcon#scale(float)} doesn't break the contract and scales correctly.
+ *
+ * @author tav
+ */
+public class IconScaleTest {
+
+  private static boolean initialSvgProp;
+
+  @Before
+  public void setState() {
+    RegistryValue rv = Registry.get("ide.svg.icon");
+    initialSvgProp = rv.asBoolean();
+    rv.setValue(true);
+
+    AbstractPainter2DTest.ScaleState.set();
+  }
+
+  @After
+  public void restoreState() {
+    Registry.get("ide.svg.icon").setValue(initialSvgProp);
+
+    AbstractPainter2DTest.ScaleState.restore();
+  }
+
+  @Test
+  public void test() throws MalformedURLException {
+    Assume.assumeFalse("Linux doesn't support JRE-HiDPI yet", SystemInfo.isLinux);
+    test(1.0);
+    test(2.0);
+    test(2.5);
+  }
+
+  public void test(double sysScale) throws MalformedURLException {
+    final int BASE_SIZE = 16;
+    final float ICON_SCALE = 1.75f;
+
+
+
+    PaintUtilTest.overrideJreHiDPIEnabled(true);
+    JBUI.setUserScaleFactor(1);
+
+    CachedImageIcon icon = new CachedImageIcon(new File(getIconPath()).toURI().toURL());
+    icon.updateScale(SYS_SCALE.of(sysScale));
+    ScaleContext originalCtx = icon.getScaleContext().copy();
+
+    TestCase.assertEquals("unexpected icon user width", BASE_SIZE, icon.getIconWidth());
+    TestCase.assertEquals("unexpected icon user height", BASE_SIZE, icon.getIconHeight());
+    TestCase.assertEquals("unexpected icon real width", 0,
+                          Double.compare(BASE_SIZE * sysScale, ImageUtil.getRealWidth(IconUtil.toImage(icon))));
+    TestCase.assertEquals("unexpected icon real height", 0,
+                          Double.compare(BASE_SIZE * sysScale, ImageUtil.getRealHeight(IconUtil.toImage(icon))));
+
+    Icon scaledIcon = icon.scale(ICON_SCALE);
+    TestCase.assertFalse("new instance of the icon is expected", icon == scaledIcon);
+    TestCase.assertEquals("ScaleContext of the original icon changed", originalCtx, icon.getScaleContext());
+
+    TestCase.assertEquals("unexpected icon user width", 0,
+                          Double.compare(BASE_SIZE * ICON_SCALE, scaledIcon.getIconWidth()));
+    TestCase.assertEquals("unexpected icon user height", 0,
+                          Double.compare(BASE_SIZE * ICON_SCALE, scaledIcon.getIconHeight()));
+    TestCase.assertEquals("unexpected icon real width", 0,
+                          Double.compare(BASE_SIZE * ICON_SCALE * sysScale, ImageUtil.getRealWidth(IconUtil.toImage(scaledIcon))));
+    TestCase.assertEquals("unexpected icon real height", 0,
+                          Double.compare(BASE_SIZE * ICON_SCALE * sysScale, ImageUtil.getRealHeight(IconUtil.toImage(scaledIcon))));
+  }
+
+  private String getIconPath() {
+    return PlatformTestUtil.getPlatformTestDataPath() + "ui/abstractClass.svg";
+  }
+}

platform/util/src/com/intellij/openapi/util/IconLoader.java

@@ -567,9 +567,10 @@ public final class IconLoader {
        * Retrieves the orig icon scaled by the provided scale.
        */
       ImageIcon getOrScaleIcon(final float scale) {
-        updateScale(OBJ_SCALE.of(scale));
+        final ScaleContext ctx = (scale == 1 ? getScaleContext() : (ScaleContext)getScaleContext().copy()); // not modifying this scale context
+        if (scale != 1) ctx.update(OBJ_SCALE.of(scale));
 
-        ImageIcon icon = SoftReference.dereference(scaledIconsCache.get(getScale(PIX_SCALE)));
+        ImageIcon icon = SoftReference.dereference(scaledIconsCache.get(ctx.getScale(PIX_SCALE)));
         if (icon != null) {
           return icon;
         }
@@ -578,17 +579,17 @@ public final class IconLoader {
           image = doWithTmpRegValue("ide.svg.icon", true, new Callable<Image>() {
             @Override
             public Image call() {
-              return loadFromUrl(getScaleContext());
+              return loadFromUrl(ctx);
             }
           });
         }
         else {
-          image = loadFromUrl(getScaleContext());
+          image = loadFromUrl(ctx);
         }
         icon = checkIcon(image, myUrl);
 
         if (icon != null && icon.getIconWidth() * icon.getIconHeight() * 4 < ImageLoader.CACHED_IMAGE_MAX_SIZE) {
-          scaledIconsCache.put(getScale(PIX_SCALE), new SoftReference<ImageIcon>(icon));
+          scaledIconsCache.put(ctx.getScale(PIX_SCALE), new SoftReference<ImageIcon>(icon));
         }
         return icon;
       }

platform/util/src/com/intellij/util/ui/JBUI.java

@@ -743,6 +743,13 @@ public class JBUI {
     private BaseScaleContext() {
     }
 
+    /**
+     * Creates a context with all scale factors set to 1.
+     */
+    public static BaseScaleContext createIdentity() {
+      return create(USR_SCALE.of(1));
+    }
+
     /**
      * Creates a context with the provided scale factors (system scale is ignored)
      */
@@ -876,6 +883,18 @@ public class JBUI {
       }
       return true;
     }
+
+    public <T extends BaseScaleContext> T copy() {
+      BaseScaleContext ctx = createIdentity();
+      ctx.updateAll(this);
+      //noinspection unchecked
+      return (T)ctx;
+    }
+
+    @Override
+    public String toString() {
+      return usrScale + ", " + objScale + ", " + pixScale;
+    }
   }
 
   /**
@@ -1037,6 +1056,19 @@ public class JBUI {
         compRef.clear();
       }
     }
+
+    @Override
+    public <T extends BaseScaleContext> T copy() {
+      ScaleContext ctx = createIdentity();
+      ctx.updateAll(this);
+      //noinspection unchecked
+      return (T)ctx;
+    }
+
+    @Override
+    public String toString() {
+      return usrScale + ", " + sysScale + ", " + objScale + ", " + pixScale;
+    }
   }
 
   /**