You can clone with
HTTPS or Subversion.
This bug applies to the in-development popup widget discussed in issue #3218, and the exact behavior varies by browser.
If the link that triggers a popup is positioned more than about 865px (depending on the browser) to the left -- whether because of its own styles or those of an ancestor element -- the resulting popup will have a negative calculated left offset. For some reason, this can be overcome by giving .ui-popup-container a max-width of less than 100%.
On the test page linked above, the following popup-triggering buttons have the problem in the browsers I've tested so far (all on Windows 7):
I saw the problems on FF 12 on Windows. It seems that sometimes the browser decides to wrap the text and at other times it does not. However, I'm not sure how far the implementation of popup should go. Currently, it makes absolutely no sizing decisions. It merely checks whether a max-size is set, but doesn't actually size the popup. The width and height of the popup are determined entirely by its contents.
Thus, this bug may indicate the need to assign a max-width to the payload div, or override the .ui-popup-container class with a custom style that adds a max-width. I'm not sure that /all/ popups should have a max-width set.
I added a max-width: 90%; to the popup container css. That should fix this and also help #4266. Please let me know if this is not the case.