Shift + Scroll Wheel does not scroll space horizontally

I don't know how to describe this well, but I'll try:

You can't scroll the LMMS window horizontally with shift+scroll wheel (this works in the song editor, though...)
The only way I can scroll the window horizontally is with the scroll bar at the bottom or with a trackpad.

(and just in case anyone's confused about "Scrolling the LMMS Window", I mean when an embedded window goes off-screen, and the scroll bars appear in the main window.)

(I am running Windows 10 1903 w/ LMMS 1.2.0)

Reproduce the issue in LMMS 1.2.0 on Windows 10 1903, comparing horizontal Shift+scroll behavior in the main window with the song editor. Trace the main window's embedded-window scrolling behavior; done means Shift+scroll wheel moves the main window horizontally without requiring the bottom scrollbar or a trackpad.

Written by the indexing model from the issue text.