Allow webview guests to be resized manually

This adds the `disableguestresize` property for webviews to prevent the
webview guest from reacting to size changes of the webview element. This
also partially documents the `webContents.setSize` function in order to
manually control the webview guest size.

These two features can be combined to improve resize performance for
e.g. webviews that span the entire window. This greatly reduces the lag
described in #6905.

spec/webview-spec.js

@@ -2,11 +2,11 @@ const assert = require('assert')
 const path = require('path')
 const http = require('http')
 const url = require('url')
-const {app, session, getGuestWebContents, ipcMain, BrowserWindow} = require('electron').remote
+const {app, session, getGuestWebContents, ipcMain, BrowserWindow, webContents} = require('electron').remote
 const {closeWindow} = require('./window-helpers')
 
 describe('<webview> tag', function () {
-  this.timeout(20000)
+  this.timeout(60000)
 
   var fixtures = path.join(__dirname, 'fixtures')
 
@@ -1346,4 +1346,114 @@ describe('<webview> tag', function () {
       document.body.appendChild(div)
     })
   })
+
+  describe('disableguestresize attribute', () => {
+    it('does not have attribute by default', () => {
+      document.body.appendChild(webview)
+      assert(!webview.hasAttribute('disableguestresize'))
+    })
+
+    it('resizes guest when attribute is not present', done => {
+      w = new BrowserWindow({ show: false, width: 200, height: 200 })
+      w.loadURL('file://' + fixtures + '/pages/webview-guest-resize.html')
+
+      w.webContents.once('did-finish-load', () => {
+        const CONTENT_SIZE = 300
+
+        const elementResizePromise = new Promise(resolve => {
+          ipcMain.once('webview-element-resize', (event, width, height) => {
+            assert.equal(width, CONTENT_SIZE)
+            resolve()
+          })
+        })
+
+        const guestResizePromise = new Promise(resolve => {
+          ipcMain.once('webview-guest-resize', (event, width, height) => {
+            assert.equal(width, CONTENT_SIZE)
+            resolve()
+          })
+        })
+
+        Promise.all([elementResizePromise, guestResizePromise]).then(() => done())
+
+        w.setContentSize(CONTENT_SIZE, CONTENT_SIZE)
+      })
+    })
+
+    it('does not resize guest when attribute is present', done => {
+      w = new BrowserWindow({ show: false, width: 200, height: 200 })
+      w.loadURL('file://' + fixtures + '/pages/webview-no-guest-resize.html')
+
+      w.webContents.once('did-finish-load', () => {
+        const CONTENT_SIZE = 300
+
+        const elementResizePromise = new Promise(resolve => {
+          ipcMain.once('webview-element-resize', (event, width, height) => {
+            assert.equal(width, CONTENT_SIZE)
+            resolve()
+          })
+        })
+
+        const noGuestResizePromise = new Promise(resolve => {
+          const onGuestResize = (event, width, height) => {
+            assert(false, 'unexpected guest resize message')
+          }
+          ipcMain.once('webview-guest-resize', onGuestResize)
+
+          setTimeout(() => {
+            ipcMain.removeListener('webview-guest-resize', onGuestResize)
+            resolve()
+          }, 500)
+        })
+
+        Promise.all([elementResizePromise, noGuestResizePromise]).then(() => done())
+
+        w.setContentSize(CONTENT_SIZE, CONTENT_SIZE)
+      })
+    })
+
+    it('dispatches element resize event even when attribute is present', done => {
+      w = new BrowserWindow({ show: false, width: 200, height: 200 })
+      w.loadURL('file://' + fixtures + '/pages/webview-no-guest-resize.html')
+
+      w.webContents.once('did-finish-load', () => {
+        const CONTENT_SIZE = 300
+
+        ipcMain.once('webview-element-resize', (event, width, height) => {
+          assert.equal(width, CONTENT_SIZE)
+          done()
+        })
+
+        w.setContentSize(CONTENT_SIZE, CONTENT_SIZE)
+      })
+    })
+
+    it('can be manually resized with setSize even when attribute is present', done => {
+      w = new BrowserWindow({ show: false, width: 200, height: 200 })
+      w.loadURL('file://' + fixtures + '/pages/webview-no-guest-resize.html')
+
+      w.webContents.once('did-finish-load', () => {
+        const GUEST_WIDTH = 10
+        const GUEST_HEIGHT = 20
+
+        ipcMain.once('webview-guest-resize', (event, width, height) => {
+          assert.equal(width, GUEST_WIDTH)
+          assert.equal(height, GUEST_HEIGHT)
+          done()
+        })
+
+        for (let wc of webContents.getAllWebContents()) {
+          if (wc.hostWebContents &&
+              wc.hostWebContents.id === w.webContents.id) {
+            wc.setSize({
+              normal: {
+                width: GUEST_WIDTH,
+                height: GUEST_HEIGHT
+              }
+            })
+          }
+        }
+      })
+    })
+  })
 })