Multi-selection of text in text areas (Chrome, Sparrow) will switch keyboard to "Insert mode" and overwrite too much text

OSX 10.7.4, MacBook Air

I've noticed that when I select text and attempt to replace it with additional text, some text areas will maintain the selection and cause me to errantly delete data from my text area. I can easily reproduce it in Chrome 19 or Sparrow 1.5, but I cannot reproduce this behavior in TextEdit or Textmate, however.

To reproduce:

1. Type some text in a text area in Chrome or Sparrow: "This is a test"
2. Select "This is"
3. "This is" will be highlighted
4. Start typing. "This is" will immediately disappear, but the selection will remain and instead highlight " a test". I hit another keystroke, and it deletes the "a test", deselects, and goes back to normal.
5. Sometimes, however, it will remain selected for an indeterminate amount of time and I'll have inadvertently deleted most of a paragraph simply by typing a single word!

It seemingly does not work if I highlight a single word - it has to do with multi-word select and typing immediately after selecting.

This is kind of insane, bizarro behavior I've never seen from OS X. Any thoughts? It's driving me crazy. I've reproduced it with the built-in trackpad and the Magic Trackpad.

For reference, it's the same issue as reported here, yet unresolved (https://discussions.apple.com/thread/3756111?start=0&tstart=0)

I think this behaviour can be attributed to Mac OS X's momentum scrolling. While you select the text the text is still being selected (due to the initial momentum of the mouse you used to select it). I can't be sure but this same thing keeps happening to me