Multi File Upload - changing window width triggers file upload
(see attached video)

After fast debugging:
Problem is caused by re-rendering of Sub-items, which are re-rendered because there is ResizeObserwer on Sub-items container div.
It should not occur on previous versions (prior to v4.0) because back then Sub-items were re-rendered on Content Tree resize event, which cannot be triggered while MFU is opened because of popup backdrop, nonetheless problem should be rather fixed also on previous MFU versions.