Set contenteditable=plaintext-only directly via an attribute

So that it isn't enabled when editable is false.

FIX: Fix a regression where `EditorView.editable.of(false)` didn't disable
editing on Webkit-based browsers.

Issue codemirror/dev#526

src/dom.ts

@@ -218,3 +218,16 @@ export function dispatchKey(elt: HTMLElement, name: string, code: number): boole
   elt.dispatchEvent(up)
   return down.defaultPrevented || up.defaultPrevented
 }
+
+let _plainTextSupported: boolean | null = null
+export function contentEditablePlainTextSupported() {
+  if (_plainTextSupported == null) {
+    _plainTextSupported = false
+    let dummy = document.createElement("div")
+    try {
+      dummy.contentEditable = "plaintext-only"
+      _plainTextSupported = dummy.contentEditable == "plaintext-only"
+    } catch(_) {}
+  }
+  return _plainTextSupported
+}

src/editorview.ts

@@ -6,7 +6,7 @@ import {StyleModule, StyleSpec} from "style-mod"
 import {DocView} from "./docview"
 import {ContentView} from "./contentview"
 import {InputState} from "./input"
-import {Rect, focusPreventScroll, flattenRect} from "./dom"
+import {Rect, focusPreventScroll, flattenRect, contentEditablePlainTextSupported} from "./dom"
 import {posAtCoords, moveByChar, moveToLineBoundary, byGroup, moveVertically, skipAtoms} from "./cursor"
 import {BlockInfo} from "./heightmap"
 import {ViewState} from "./viewstate"
@@ -355,7 +355,7 @@ export class EditorView {
       spellcheck: "false",
       autocorrect: "off",
       autocapitalize: "off",
-      contenteditable: String(this.state.facet(editable)),
+      contenteditable: !this.state.facet(editable) ? "false" : contentEditablePlainTextSupported() ? "plaintext-only" : "true",
       class: "cm-content",
       style: `${browser.tabSize}: ${this.state.tabSize}`,
       role: "textbox",

src/theme.ts

@@ -58,7 +58,6 @@ export const baseTheme = buildTheme("." + baseThemeID, {
     display: "block",
     whiteSpace: "pre",
     wordWrap: "normal", // https://github.com/codemirror/codemirror.next/issues/456
-    WebkitUserModify: "read-write-plaintext-only",
     boxSizing: "border-box",
 
     padding: "4px 0",