Adding a new task registered as 2 transactions when computer is slow

[pmiklashevych]

http://lh/master/bryntum-suite/gantt/examples/undoredo/

Slow down your machine (dev-tools -> performance -> CPU -> 6x slower)
Add a task in console:
gantt.taskStore.rootNode.appendChild({ name : 'New task', duration : 1 })
See 2 transactions registered (and 2 badge is displayed on the undo button)

Reload the page and repeat with no throttling. See one transaction is registered

It affects http://lh/master/bryntum-suite/gantt/tests/#examples/php.t.js?es6
It sometimes fails with

Undo/Redo should be clear clear after load
    fail 5 - Waited too long for:  selector ".b-badge[data-ref="undoBtn"][data-badge="1"]" to appear
    Failed assertion `waitForSelector` 
    Condition was not fullfilled during 30000ms.