Bug (v9.4) Split pane left no longer possible

[IanOfYork]

Dragging a document tab to the right side of a pane, shows feedback when the mouse is over half way across.
Releasing the mouse then splits the pane and drops the document into the right side.

Dragging a document tab to the left side of a pane, does not show any feedback.
Releasing the mouse then splits the pane and drops the document into the right side.
In version 8, this would drop the document into the left side.