Comment 26 for bug 677551

So a bit of testing with a 4-line textarea....

I confirmed that typing non-newline characters reflows just the line they're typed on (as long as it doesn't wrap) while typing enter reflows all the lines.

I also confirmed that typing enter a single time clears textruns once via that loop and reflows each line exactly once.

I also confirmed that cutting and then pasting all the text in the textarea reflows each line exactly once. Also that it's much much faster than hitting enter.

I added a printf in ClearAllTextRunReferences right before the SetTextRun call, like so:

    if (aFrame->GetContent()->GetParent() &&
        aFrame->GetContent()->GetParent()->GetParent() &&
        aFrame->GetContent()->GetParent()->GetParent()->Tag() ==
          nsGkAtoms::textarea &&
        aFrame->GetTextRun()) {
      printf("Clearing: %p\n", aFrame);
    }
and one at the beginning of nsTextFrame::ReflowText like so:

  if (mContent->GetParent() &&
      mContent->GetParent()->GetParent() &&
      mContent->GetParent()->GetParent()->Tag() == nsGkAtoms::textarea) {
    printf("Reflowing: %p\n", this);
  }

Here's what the output looks like if I paste 4 lines into a textarea (the lines contain one char each: "a", "b", "c", and "d"):

  Reflowing: 0x103bd8990
  Reflowing: 0x103bd93a8
  Reflowing: 0x103bd92f0
  Reflowing: 0x103bd9198

Here's what happens if I hit space on the first line before the "a":
  Clearing: 0x103bd8990
  Clearing: 0x103bd93a8
  Clearing: 0x103bd92f0
  Clearing: 0x103bd9198
  Reflowing: 0x103bd8990Here's what happens if I hit enter before the "d":

  Clearing: 0x103bd9198
  Reflowing: 0x103bd92f0
  Reflowing: 0x103bd9198
  Reflowing: 0x1041b06d8
Here's what happens if I hit enter before the "c":

  Clearing: 0x103bd92f0
  Clearing: 0x103bd9198
  Reflowing: 0x103bd93a8
  Reflowing: 0x103bd92f0
  Clearing: 0x103bd92f0
  Clearing: 0x103bd9198
  Reflowing: 0x103bd9198
  Reflowing: 0x1041b06d8Here's what I get if I hit enter before the "a":

  Clearing: 0x103bd8990
  Clearing: 0x103bd93a8
  Clearing: 0x103bd92f0
  Clearing: 0x103bd9198
  Reflowing: 0x103bd8990
  Clearing: 0x103bd8990
  Clearing: 0x103bd93a8
  Reflowing: 0x103bd93a8
  Clearing: 0x103bd93a8
  Clearing: 0x103bd92f0
  Reflowing: 0x103bd92f0
  Clearing: 0x103bd92f0
  Clearing: 0x103bd9198
  Reflowing: 0x103bd9198
  Reflowing: 0x1041b06d8
So it looks like we clear all the textruns for the textarea from the CharacterDataChanged notification, then reflow all the lines, and before reflowing each line we clear its textrun, which is non-null at that point. So presumably we're constructing all those textruns twice... That still doesn't quite explain the behavior I see, where the difference is a lot more than 2x between paste and hitting enter.