Created attachment 171673 [details]
Document with horizontal and vertical line

If a line is horizontal or vertical in pptx, they are not horizontal or vertical after opening the file in Impress. The angles are slightly off. You get 359.99deg for a horizontal line and 270.01deg for a vertical line for example.

Besides looking at the rotation angle in the Position&Size dialog, you will notice the error in the status bar, if the line is selected. The shown text should include "horizontal" or "vertical", but it doesn't.

This slight deviation cancels out the horizontal/vertical detection, which is used in several places in the code.

The error is likely in the part https://opengrok.libreoffice.org/xref/core/oox/source/drawingml/shape.cxx?r=2a70cfb0#844
The handling of lines there is questionable:
* A line has exactly two points and one polygon. No need for a loop.
* The 'Guard against zero width or height' is wrong. A vertical line has zero width and a horizontal line has zero height. The current solution prevents vertical and horizontal lines. Even height and width zero at the time is possible. Such zero length lines are visible, if line caps are set.
* What is the purpose of 'xModelInfo' there?

But I have not tested yet, whether a fix there would solve the problem.