unexpected refresh of a webview

Bug Description:

A webview is been refreshed when it is maximized or minimized

Steps to Reproduce:

1. download and run https://github.com/microsoft/vscode-extension-samples/tree/master/webview-sample
2. when you maximize or minimize the running webivew you see that the content of a webview is disappears for a few seconds, actually it is being refreshed and it does not stop in any break point even if you put them in a vscode webvew life cycle methods.

Additional Information

@theia/core 1.3.0