As of October 2020 this is now back in the designer.
Reported on April 1st 2019, the tree view width in the Power Apps designer no longer allows adjustment of width. As noted in that thread it's uncertain why an option like that would be removed. With using best practices on naming to easily find controls they are often too wide for the space and you have to use the slider control at the bottom of the tree view that is half transparent and not the easiest to use. When you rename a control it often shifts the tree view window so that you can no longer see the beginning of the data card name requiring to use the slider to move the tree view so the name is visible. When you're renaming 50 data card value controls so you can identify them in formulas it's extremely cumbersome, frustrating and adds a lot of time to just renaming controls.
Using the browser zoom controls doesn't even work as the tree view scales with the zoom changes so the relative width is the same.