Difference between taskbar shown in screenshot and display + grey line problem

[Ignitus]

Hi, I will go straight to the point of the issue. My laptop (ProBook 450 G5)'s display sporadically (sometimes after few minutes I turn it on, sometimes after hours of usage) flickers once or twice in a row (the right end of the image I am using as desktop background briefly appears at the left of my screen, like overlapping with the actual background), then a grey line appears below my taskbar, in correspondence of the place where the running apps highlighters are located (check pictures to see what I am talking about). This basically makes impossible to see which apps are running and is very annoying overall. I realized that if I move the cursor on the extreme top of the screen, it appears miniaturized in the grey line in question, like the line is a continuation of the top part of the desktop. Moreover, if I take a screenshot of my desktop it is perfect (running apps and highlighters perfectly visible), like the issue does not exist at all. I installed all the updates available from both Windows, Intel and HP websites and totally formatted the disk twice, without being able to solve this issue. What else could I possibly do? By the way, I recently changed my damaged hard disk with a SSD, and seems to me that the issue has been appearing  since then. Thanks for your attention.  

[Ignitus]

Pictures to help you understand what I am talking about:

The normal state of the taskbar, as it appears in the screenshot even when the issue appears

The actual thing I see on the screen when the issue appears