Ignore clicks on block decorations

Previously, clicking on a block decoration to interact with it would
cause the editor to scroll to the line next to it. This is inconvenient,
especially if the decoration was designed to be interactive and
contained buttons or links. If the decoration was close to the bottom of
the screen, clicking on a button inside of it would make the editor
scroll down and abort the click.

This behavior regressed during the editor rendering layer rewrite and
with this commit we are restoring the original behavior by simply
ignoring clicks that land on block decorations.

spec/text-editor-component-spec.js

@@ -2153,6 +2153,39 @@ describe('TextEditorComponent', () => {
       expect(component.refs.blockDecorationMeasurementArea.offsetWidth).toBe(component.getScrollWidth())
     })
 
+    it('does not change the cursor position when clicking on a block decoration', async () => {
+      const {editor, component} = buildComponent()
+
+      const decorationElement = document.createElement('div')
+      decorationElement.textContent = 'Parent'
+      const childElement = document.createElement('div')
+      childElement.textContent = 'Child'
+      decorationElement.appendChild(childElement)
+      const marker = editor.markScreenPosition([4, 0])
+      editor.decorateMarker(marker, {type: 'block', item: decorationElement})
+      await component.getNextUpdatePromise()
+
+      const decorationElementClientRect = decorationElement.getBoundingClientRect()
+      component.didMouseDownOnContent({
+        target: decorationElement,
+        detail: 1,
+        button: 0,
+        clientX: decorationElementClientRect.left,
+        clientY: decorationElementClientRect.top
+      })
+      expect(editor.getCursorScreenPosition()).toEqual([0, 0])
+
+      const childElementClientRect = childElement.getBoundingClientRect()
+      component.didMouseDownOnContent({
+        target: childElement,
+        detail: 1,
+        button: 0,
+        clientX: childElementClientRect.left,
+        clientY: childElementClientRect.top
+      })
+      expect(editor.getCursorScreenPosition()).toEqual([0, 0])
+    })
+
     function createBlockDecorationAtScreenRow(editor, screenRow, {height, margin, position}) {
       const marker = editor.markScreenPosition([screenRow, 0], {invalidate: 'never'})
       const item = document.createElement('div')