support precise wheel rotation to scroll a viewport

platform/platform-api/src/com/intellij/ui/components/JBScrollPane.java

@@ -182,7 +182,16 @@ public class JBScrollPane extends SmoothScrollPane {
                 JScrollPane pane = (JScrollPane)source;
                 if (pane.isWheelScrollingEnabled()) {
                   JScrollBar bar = event.isShiftDown() ? pane.getHorizontalScrollBar() : pane.getVerticalScrollBar();
-                  if (bar != null && bar.isVisible()) oldListener.mouseWheelMoved(event);
+                  if (bar != null && bar.isVisible()) {
+                    boolean isUnitScroll = MouseWheelEvent.WHEEL_UNIT_SCROLL == event.getScrollType();
+                    JViewport viewport = pane.getViewport();
+                    if (isUnitScroll && viewport instanceof JBViewport && isPreciseRotationSupported()) {
+                      ((JBViewport)viewport).updateViewPosition(event.isShiftDown(), 10 * event.getPreciseWheelRotation());
+                    }
+                    else {
+                      oldListener.mouseWheelMoved(event);
+                    }
+                  }
                 }
               }
             }
@@ -735,7 +744,7 @@ public class JBScrollPane extends SmoothScrollPane {
     if (event.isConsumed()) return false;
     // any rotation expected (forward or backward)
     boolean ignore = event.getWheelRotation() == 0;
-    if (ignore && isPreciseRotationSupported()) {
+    if (ignore && (isPreciseRotationSupported() || isTrueSmoothScrollingEnabled())) {
       double rotation = event.getPreciseWheelRotation();
       double delta = MouseWheelEventEx.getScrollingDelta(event);
       ignore = (rotation == 0.0D || !Double.isFinite(rotation)) && (delta == 0.0D || !Double.isFinite(delta));
@@ -743,8 +752,15 @@ public class JBScrollPane extends SmoothScrollPane {
     return !ignore && 0 == (SCROLL_MODIFIERS & event.getModifiers());
   }
 
-  private static boolean isPreciseRotationSupported() {
-    return isTrueSmoothScrollingEnabled();
+  /**
+   * Indicates whether the smooth scrolling is supported by any means.
+   *
+   * @deprecated will be removed after fixing a blit-scrolling
+   */
+  @Deprecated
+  @SuppressWarnings("DeprecatedIsStillUsed")
+  public static boolean isPreciseRotationSupported() {
+    return Registry.is("ide.scroll.precise");
   }
 
   private static final int SCROLL_MODIFIERS = // event modifiers allowed during scrolling

platform/platform-api/src/com/intellij/ui/components/JBViewport.java

@@ -79,6 +79,10 @@ public class JBViewport extends JViewport implements ZoomableViewport {
 
   private volatile boolean myBackgroundRequested; // avoid cyclic references
 
+  private boolean myUpdateViewPosition; // avoid cyclic references
+  private double myViewX;
+  private double myViewY;
+
   public JBViewport() {
     addContainerListener(new ContainerListener() {
       @Override
@@ -107,9 +111,42 @@ public class JBViewport extends JViewport implements ZoomableViewport {
       checkScrollingCapabilities();
     }
 
+    if (myUpdateViewPosition) return;
+    myViewX = p.x;
+    myViewY = p.y;
     super.setViewPosition(p);
   }
 
+  /**
+   * Updates a view position directly without using a corresponding scroll bar.
+   *
+   * @param horizontal   {@code true} for horizontal scrolling, {@code false} for vertical scrolling
+   * @param preciseValue precise wheel rotation in pixels for the specified direction
+   */
+  void updateViewPosition(boolean horizontal, double preciseValue) {
+    int x = (int)myViewX;
+    int y = (int)myViewY;
+    if (horizontal) {
+      int old = x;
+      myViewX = Math.max(0, myViewX + preciseValue);
+      x = (int)myViewX;
+      if (x == old) return; // nothing changed
+    }
+    else {
+      int old = y;
+      myViewY = Math.max(0, myViewY + preciseValue);
+      y = (int)myViewY;
+      if (y == old) return; // nothing changed
+    }
+    try {
+      myUpdateViewPosition = true;
+      super.setViewPosition(new Point(x, y));
+    }
+    finally {
+      myUpdateViewPosition = false;
+    }
+  }
+
   // A heuristic to detect whether this viewport belongs to the "Event Log" tool window (which we use for output)
   private boolean isInsideLogToolWindow() {
     Container parent1 = getParent();

platform/platform-impl/src/com/intellij/openapi/editor/impl/EditorImpl.java

@@ -507,7 +507,8 @@ public final class EditorImpl extends UserDataHolderBase implements EditorEx, Hi
     myPanel = new JPanel();
 
     // JBScrollPane.Layout relies on "opaque" property directly (instead of "editor.transparent.scrollbar")
-    if (SystemProperties.isTrueSmoothScrollingEnabled() && !IdeBackgroundUtil.isBackgroundImageSet(project)) {
+    boolean opaque = JBScrollPane.isPreciseRotationSupported() || SystemProperties.isTrueSmoothScrollingEnabled();
+    if (opaque && !IdeBackgroundUtil.isBackgroundImageSet(project)) {
       //Do not set opaque to false if a scroll bar is opaque (System Preferences / Show scroll bars / Always)
       myVerticalScrollBar.setOpaque(true);
       myScrollPane.getHorizontalScrollBar().setOpaque(true);

platform/platform-impl/src/com/intellij/openapi/editor/impl/EditorMarkupModelImpl.java

@@ -54,6 +54,7 @@ import com.intellij.openapi.util.text.StringUtil;
 import com.intellij.openapi.wm.impl.IdeBackgroundUtil;
 import com.intellij.ui.*;
 import com.intellij.ui.awt.RelativePoint;
+import com.intellij.ui.components.JBScrollPane;
 import com.intellij.util.Alarm;
 import com.intellij.util.SystemProperties;
 import com.intellij.util.containers.ContainerUtil;
@@ -487,7 +488,8 @@ public class EditorMarkupModelImpl extends MarkupModelImpl implements EditorMark
        This helps to improve scrolling performance and to reduce CPU usage (especially if drawing is compute-intensive).
 
        When there's a background image, blit-acceleration cannot be used (because of the static overlay). */
-    return !(SystemProperties.isTrueSmoothScrollingEnabled() && !IdeBackgroundUtil.isBackgroundImageSet(myEditor.getProject())) &&
+    boolean opaque = JBScrollPane.isPreciseRotationSupported() || SystemProperties.isTrueSmoothScrollingEnabled();
+    return !(opaque && !IdeBackgroundUtil.isBackgroundImageSet(myEditor.getProject())) &&
            Registry.is("editor.transparent.scrollbar", false) &&
            EditorUtil.isRealFileEditor(myEditor);
   }

platform/util/resources/misc/registry.properties

@@ -118,6 +118,9 @@ ide.splitter.mouseZone=6
 ide.autoscroll.from.source.delay=100
 ide.autoscroll.from.source.delay.description=Delay before performing the AutoscrollFromSource action
 
+ide.scroll.precise=false
+ide.scroll.precise.description=Use precise wheel rotation (may decrease scrolling performance).
+
 ide.scroll.default.unit.increment=false
 ide.scroll.default.unit.increment.description=Use default unit increment from Java (may decrease scrolling performance).