I attached a patch for defect [ #10531: vf_drawtext causing font 
rendering jitter after libharfbuz commit. ]

Before the patch the width of a text line was measured up to the 
rightmost visible pixel of the last character.
Pros: the right padding specified with the boxborderw parameter is exact 
with respect to the visible text
Cons: the width of a text line depends on the last character, even with 
monospaced fonts

After the patch the width of the last character is set equal to the 
width specified in the font file, which may include some empty space.
Pros: when using monospaced fonts the width of a line depends only on 
the number of characters, this is also the standard behavior adopted by 
other rendering strategies, e.g. in browsers
Cons: the user specified right padding may not be equal to the distance 
between the background box right border and the rightmost visible pixel 
of the text.