Can't grab the horizontall scrollbar when the cursor is hidden on big cells

On mode editor, working on big cells (larger than the window height), If the cursor is on the big line and I go for the horizontall scrollbar, clicking on it for grabbing to move, it returns me to the cursor focus and can't move the horizontall scrollbar.

Reproduce the issue in the mode editor with a cell larger than the window height and the cursor on the large line. Trace the editor's horizontal scrollbar focus and drag handling; done when the scrollbar can be grabbed and moved without returning focus to the cursor.

Written by the indexing model from the issue text.