The story about line wrapping is mainly that, you:

  · Break the text into runs.
  · Calculate BiDi at paragraph level.
  · Shape every run to get its length and possible line breaking points.
  · Arrange the runs (now only have length, height, text isnide it and some breaking properties — glyphs and glyph positions are dropped) into lines.

Such process is called “measuring”. And the real text display is delayed until a line become visible: you fetch out the text runs in the line and do the real display.

In most times (like appending text) you do not to do much recalculation.