treeviewcolumn: Clip header buttons
We don't want to overdraw when dragging a narrow column around, and we also need the clipping to avoid picking the wrong column, when a later column button overlaps an earlier one.
Fixes: #4045 (closed)
We don't want to overdraw when dragging a narrow column around, and we also need the clipping to avoid picking the wrong column, when a later column button overlaps an earlier one.
Fixes: #4045 (closed)