Add EditorView.scrollHandler

FEATURE: The `EditorView.scrollHandler` facet can be used to override or extend
the behavior of the editor when things are scrolled into view.

Issue codemirror/dev#1355

src/docview.ts

@@ -9,7 +9,7 @@ import {getAttrs} from "./attributes"
 import {clientRectsFor, isEquivalentPosition, maxOffset, Rect, scrollRectIntoView,
         getSelection, hasSelection, textRange, DOMSelectionState} from "./dom"
 import {ViewUpdate, decorations as decorationsFacet, outerDecorations,
-        ChangedRange, ScrollTarget, getScrollMargins} from "./extension"
+        ChangedRange, ScrollTarget, scrollHandler, getScrollMargins, logException} from "./extension"
 import {EditorView} from "./editorview"
 import {Direction} from "./bidi"
 
@@ -558,6 +558,11 @@ export class DocView extends ContentView {
       return
     }
 
+    for (let handler of this.view.state.facet(scrollHandler)) {
+      try { if (handler(this.view, target.range, target)) return true }
+      catch(e) { logException(this.view.state, e, "scroll handler") }
+    }
+
     let {range} = target
     let rect = this.coordsAt(range.head, range.empty ? range.assoc : range.head > range.anchor ? -1 : 1), other
     if (!rect) return

src/editorview.ts

@@ -16,7 +16,7 @@ import {ViewUpdate, styleModule,
         exceptionSink, updateListener, logException,
         viewPlugin, ViewPlugin, PluginValue, PluginInstance, decorations, outerDecorations, atomicRanges,
         scrollMargins, MeasureRequest, editable, inputHandler, focusChangeEffect, perLineTextDirection,
-        scrollIntoView, UpdateFlag, ScrollTarget, bidiIsolatedRanges, getIsolatedRanges} from "./extension"
+        scrollIntoView, UpdateFlag, ScrollTarget, bidiIsolatedRanges, getIsolatedRanges, scrollHandler} from "./extension"
 import {theme, darkTheme, buildTheme, baseThemeID, baseLightID, baseDarkID, lightDarkIDs, baseTheme} from "./theme"
 import {DOMObserver} from "./domobserver"
 import {Attrs, updateAttrs, combineAttrs} from "./attributes"
@@ -945,6 +945,12 @@ export class EditorView {
   /// dispatching the custom behavior as a separate transaction.
   static inputHandler = inputHandler
 
+  /// Scroll handlers can override how things are scrolled into view.
+  /// If they return `true`, no further handling happens for the
+  /// scrolling. If they return false, the default scroll behavior is
+  /// applied. Scroll handlers should never initiate editor updates.
+  static scrollHandler = scrollHandler
+
   /// This facet can be used to provide functions that create effects
   /// to be dispatched when the editor's focus state changes.
   static focusChangeEffect = focusChangeEffect

src/extension.ts

@@ -38,6 +38,12 @@ export const nativeSelectionHidden = Facet.define<boolean, boolean>({
   combine: values => values.some(x => x)
 })
 
+export const scrollHandler = Facet.define<(
+  view: EditorView,
+  range: SelectionRange,
+  options: {x: ScrollStrategy, y: ScrollStrategy, xMargin: number, yMargin: number}
+) => boolean>()
+
 export class ScrollTarget {
   constructor(
     readonly range: SelectionRange,