In various sections of the frontend (i.e. item creation/update, trigger creation/update, template modification, etc) the text boxes are no longer resizable horizontally. They can only be resized vertically. For example, start creating an item, choose calculated and notice that the formula text box only allows for vertical resizing. The same is true for the description text box, as well as for trigger expression, recovery expression, and many other places. It looks like in the style sheets that resize: vertical was added to fix an overlap issue in ZBX-22418. Specifically in the commit found here.

I looked through the master branch and it looks like it's gone now. So maybe this will no longer be an issue once 7.0 is released? It looks like it was committed to 6.0 releases as well.