it took me days to track this bugger down. When placing a slider into an Ext.Container with other components, the slider width is rendered correctly. However, the thumb's dragObj boundary is always calculated as if there were no other components in the container, i.e. way too wide, thus moving the thumb out of visible range at higher values.

It does not matter whether or not one specifies flex layouting or a fixed width for the slider.
It does not matter whether the value is input via config "value" option or .setValue().

Quick hack to fix: """sliderElementReference.getThumb().dragObj.updateBoundary();""" after the component has rendered.