I recently upgraded IER from 2.6.5.17 to 2.6.5.18.

After sending a reply and returning to Pegasus in preview mode, the window with message text drops to a 1/4 of it's full size, complete with scroll bars horizontally and vertically.

It seems to only happen if the message that you are returning to after a reply, has html in it. All is ok if the message you are returning to is plain text.

When I select a different message or folder the message text window draws properly again.

When I downgrade IER to 2.6.5.17, the problem doesn't appear. So certainly seems to be an 2.6.5.18 issue.

It's an issue only occuring if using the inner child windows of Pegasus Mail in maximized mode (you can tell from the missing title bar of the preview window in the screenshot above where its sizing buttons top right are part of the main window's menu bar). I've already fixed it for the next IER update release.