Strange things are happening as of the latest update when using the Alt key to swap selection modes. It stops selecting anything altogether and doesn’t detect the left button being unclicked, causing the selection box to linger with no buttons pressed. Objects are highlighted while the box is active but they are not selected after left-clicking.
If you click directly on an object, it is selected and other objects that were highlighted continue to appear so. Even the object list on the left is fooled. Hovering over each object causes them to become deselected. Moving the cursor into the list area triggers deselection of all of the objects, except if you left-clicked on an object in which case it remains selected. Visual artifacts of subsequent selection boxes remain and only clear up after zooming in/out.
Version 2019-1.9 (same problem in browser)
Linux Mint 19 Xfce