Popups/tooltips in the Wikidata interface cause the page to scroll horizontally on narrower screens. It happens in various places:

The tooltip for the help icon (screenshot from https://www.wikidata.org/wiki/Q33216580 at 700px):

The license popup when editing a statement for the first time (screenshot from the same page):

The language selector for monolingual text statements (screenshot from https://www.wikidata.org/wiki/Q44112320 at 800px):

The unit selector for quantity statements (screenshot from the same page):

The preview and precision/calendar options for date statements (screenshot from the same page):

The map preview and precision options for coordinate statements (screenshot from https://www.wikidata.org/wiki/Q11120461 at 800px):

These are all caused by the popup/tooltip being positioned from to the left, even if the window is not wide enough. It should probably check whether the desired left offset plus the maximum width of the popup/tooltip is larger than the width of the window, and if it is, use right: 0 to position it instead.