Block selection broken when using tabs

I'm using tabs, not spaces, and have changed my tab width to 4, not 8. I don't know if these may or may not be the determining factor, but block selection is very broken on this test file, in the fill_value() function.

When I try to block-select before the '=' column to indent the post-'=' column of the top section correctly, it selects a squiggle through the lines below. When I try copying a column of tabs from the left side to paste into the right column, it pastes all squiggly.