v1.22 released in April, 2018 has just added the ability to add horizontal scrolling to various tree elements, like the explorer pane. Horizontal scrolling support.
// Controls whether trees support horizontal scrolling in the
workbench.
"workbench.tree.horizontalScrolling": true, (deprecated, don't use)
False is the default so you will need to change the setting. A restart is required.
UPDATE: vscode v1.32 just changed the setting to:
"workbench.list.horizontalScrolling": true,
Workbench > List: Horizontal Scrolling in the settings UI.