Applying markups to overlapping markers creates inaccurate AT

[bantic]

To recreate:

• select two characters, apply "I" markup (via toolbar button)
• select the second of the two characters and the (plain) character to its right, apply "B" markup (via toolbar)

Expected output: 1 italic, 1 bold+italic, and 1 bold character.
Actual output: 1 italic, 1 bold+italic, 1 italic character (should be bold and not italic)