Double-click a column/label to rename. The text must not move when entering edit; a blue underline just appears beneath it. Red guide lines mark the label's top/bottom — watch whether the glyphs cross them.
The 2px border-bottom with box-sizing:border-box on an 18px box lifts the glyphs off the label baseline.
Underline is a box-shadow; the input keeps the label's exact height / line-height, so glyphs never move.
Double-click the text. It swaps to the underlined input in place — the glyphs should not shift by a pixel.