Add an extension point implementing gutter icons merging

also adjust calculation of initial icon are width, to avoid gutter 'jumping' (for icon sizes up to 17px)

platform/editor-ui-api/src/com/intellij/openapi/editor/GutterMarkPreprocessor.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2000-2016 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.intellij.openapi.editor;
+
+import com.intellij.codeInsight.daemon.GutterMark;
+import com.intellij.openapi.extensions.ExtensionPointName;
+import com.sun.istack.internal.NotNull;
+
+import java.util.List;
+
+public interface GutterMarkPreprocessor {
+  ExtensionPointName<GutterMarkPreprocessor> EP_NAME = ExtensionPointName.create("com.intellij.gutterMarkPreprocessor");
+
+  @NotNull
+  List<GutterMark> processMarkers(@NotNull List<GutterMark> list);
+}

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

@@ -68,6 +68,7 @@ import gnu.trove.TIntArrayList;
 import gnu.trove.TIntFunction;
 import gnu.trove.TIntObjectHashMap;
 import gnu.trove.TIntObjectProcedure;
+import gnu.trove.TObjectFunction;
 import org.jetbrains.annotations.NonNls;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
@@ -114,6 +115,7 @@ import java.util.List;
  */
 class EditorGutterComponentImpl extends EditorGutterComponentEx implements MouseListener, MouseMotionListener, DataProvider {
   private static final Logger LOG = Logger.getInstance("#com.intellij.openapi.editor.impl.EditorGutterComponentImpl");
+  private static final int START_ICON_AREA_WIDTH = JBUI.scale(17);
   private static final int FREE_PAINTERS_LEFT_AREA_WIDTH = JBUI.scale(8);
   private static final int FREE_PAINTERS_RIGHT_AREA_WIDTH = JBUI.scale(5);
   private static final int GAP_BETWEEN_ICONS = JBUI.scale(3);
@@ -775,9 +777,23 @@ class EditorGutterComponentImpl extends EditorGutterComponentEx implements Mouse
           myLineToGutterRenderers.put(line, renderers);
         }
 
-        if (renderers.size() < 5) { // Don't allow more than 5 icons per line
-          renderers.add(renderer);
+        renderers.add(renderer);
+      }
+    });
+
+    myLineToGutterRenderers.transformValues(new TObjectFunction<List<GutterMark>, List<GutterMark>>() {
+      @Override
+      public List<GutterMark> execute(List<GutterMark> value) {
+        List<GutterMark> newValue = value;
+        for (GutterMarkPreprocessor preprocessor : GutterMarkPreprocessor.EP_NAME.getExtensions()) {
+          newValue = preprocessor.processMarkers(value);
         }
+
+        if (newValue.size() >= 5) { // Don't allow more than 5 icons per line
+          newValue = newValue.subList(0, 4);
+        }
+
+        return newValue;
       }
     });
   }
@@ -804,10 +820,11 @@ class EditorGutterComponentImpl extends EditorGutterComponentEx implements Mouse
       return;
     }
 
+    int minWidth = (int)(START_ICON_AREA_WIDTH * myEditor.getScale());
     if (canShrink) {
-      myIconsAreaWidth = myEditor.getLineHeight();
+      myIconsAreaWidth = minWidth;
     } else {
-      myIconsAreaWidth = Math.max(myIconsAreaWidth, myEditor.getLineHeight());
+      myIconsAreaWidth = Math.max(myIconsAreaWidth, minWidth);
     }
 
     processGutterRenderers((line, renderers) -> {

platform/platform-resources/src/META-INF/PlatformExtensionPoints.xml

@@ -261,6 +261,7 @@
 
     <extensionPoint name="vfs.local.pluggableFileWatcher" interface="com.intellij.openapi.vfs.local.PluggableFileWatcher" />
 
+    <extensionPoint name="gutterMarkPreprocessor" interface="com.intellij.openapi.editor.GutterMarkPreprocessor" />
     <extensionPoint name="remote.credentialsType" interface="com.intellij.remote.ext.CredentialsTypeEx"/>
     <extensionPoint name="remote.credentialsLanguageContribution"
                     interface="com.intellij.remote.ext.CredentialsLanguageContribution"/>