Feat: editable dom element (#39341)

* feat: __testEditable allows editable region tests

It's not entirely isolated, but it makes it possible to select only the
element with id 'editable-only' which is built solely from code inside
the editable region.

* fix(client): missing editableContents -> ''

Previously it was added as the string 'undefined'

* fix: more informative error messages

* fix: DRY, correct and test getLines

utils/get-lines.test.js

@@ -0,0 +1,35 @@
+/* global describe expect it */
+
+const { getLines } = require('./get-lines');
+
+const content = 'one\ntwo\nthree';
+
+describe('dasherize', () => {
+  it('returns a string', () => {
+    expect(getLines('')).toBe('');
+  });
+  it("returns '' when the second arg is empty", () => {
+    expect(getLines(content)).toBe('');
+  });
+  it("returns '' when the range is negative", () => {
+    expect(getLines(content, [1, -1])).toBe('');
+  });
+  it("returns '' when the range is [n,n]", () => {
+    expect(getLines(content, [0, 0])).toBe('');
+    expect(getLines(content, [1, 1])).toBe('');
+    expect(getLines(content, [2, 2])).toBe('');
+  });
+
+  it('returns the first line when the range is [0,2]', () => {
+    expect(getLines(content, [0, 2])).toBe('one');
+  });
+  it('returns the second line when the range is [1,3]', () => {
+    expect(getLines(content, [1, 3])).toBe('two');
+  });
+  it('returns the first and second lines when the range is [0,3]', () => {
+    expect(getLines(content, [0, 3])).toBe('one\ntwo');
+  });
+  it('returns the second and third lines when the range is [1,4]', () => {
+    expect(getLines(content, [1, 4])).toBe('two\nthree');
+  });
+});