This is opposite the conventional behaviour of pretty much every operating system, desktop environment, UI framework, application and platform. I would recommend inverting the vertical and horizontal scrolling that this commit implements. i.e. unmodified scrolling for vertical, Shift + scrolling for horizontal. Users would be far less likely to get a jarring UX that way.

At the least, this should be an application setting/preference to let the user decide the way they want Shift + scroll to behave.

Suggested patch:

diff --git a/src/timeline2/view/qml/timeline.qml b/src/timeline2/view/qml/timeline.qml
index 948b52ca6..8fb331e63 100644
--- a/src/timeline2/view/qml/timeline.qml
+++ b/src/timeline2/view/qml/timeline.qml
@@ -81,13 +81,13 @@ Rectangle {
                 root.zoomOut(true);
             }
         } else if (wheel.modifiers & Qt.ShiftModifier) {
-            // Vertical scroll
-            var newScroll = Math.min(scrollView.flickableItem.contentY - wheel.angleDelta.y, trackHeaders.height - tracksArea.height + scrollView.__horizontalScrollBar.height + cornerstone.height)
-            scrollView.flickableItem.contentY = Math.max(newScroll, 0)
-        } else {
             // Horizontal scroll
             var newScroll = Math.min(scrollView.flickableItem.contentX - wheel.angleDelta.y, timeline.fullDuration * root.timeScale - (scrollView.width - scrollView.__verticalScrollBar.width))
             scrollView.flickableItem.contentX = Math.max(newScroll, 0)
+        } else {
+            // Vertical scroll
+            var newScroll = Math.min(scrollView.flickableItem.contentY - wheel.angleDelta.y, trackHeaders.height - tracksArea.height + scrollView.__horizontalScrollBar.height + cornerstone.height)
+            scrollView.flickableItem.contentY = Math.max(newScroll, 0)
         }
         wheel.accepted = true
     }