I am not sure what technically those green lines are but I am pretty sure its an artifact on page layout after objects indexing going wrong. What I mean by this is that ArcObjects logic enumerates all items on the page and assigns order number. The logic of ArcObjects that are under any Python module in Arcgis use COM libraries that follow logic more specific to C#, Java, VBA etc. In the past, if Python tools interacted with this framework it was not always 100% compatible and sometimes accessing ArcObjects related to page view container elements and editing/inserting/removing would result in improper reindexing elements within the page container because some mechanism was not chained and did not triggered. This happened to me multiple times when I was developing Python solutions with Comtypes that operated on ArcObjects libraries (I know long time ago- no need to do this now since arcpy api gets better). What I found to work in individual MXD was to select all page elements COPY and PAST (or even UNDO). After this operation elements in the page container are reindexed in the proper way by normal ArcObjects and all go back to normal. Word of warning, if you have green lines on your page layout- remove them this way or another. Broken indexing means that some page object/elements are in a wrong state and may disappear or breake you mxd.
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