To calculate string width, Konsole uses character cell widths.
Rendering engine uses actual characters widths for this purpose.
Those sizes are equal in fixed-width fonts, as long as the font
has specific character. Since no font contains all Unicode characters,
some of them are taken from different fonts and their sizes differ
from their cell size. This can lead to different results depending
on how text line is split into strings.

It can be assumed that characters in Basic Latin block in every
fixed-width font are really fixed-width, and can be rendered as one
string. All other characters are rendered separately, and their
rendering surface is limited to their cell.

Before/After screenshots:

BUG: 361547

Paste this: ⸻test in konsole and highlight different parts of the string

In a text editor paste this:

| A |
| ⸻ |
| ⟹ |
| ⧠ |

and move cursor vertically through characters column