How to reproduce:
1. Open the submitted project(ProgressBarProblem.zip)
2. From the Menu bar select Examples > Progress Bar Usage
3. Click 'Display bar'
4. Unfocus the Editor
5. Refocus the Editor by clicking anywhere in the editor except for the progress bar itself

Expected result: the progress bar is still visible
Actual result: the progress bar is hidden, its progress can still be seen in the Windows taskbar

Reproducible with: 2020.1.15f1, 2020.2.0b12, 2020.2.0b13, 2021.1.0a4, 2021.1.0a7
Not reproducible with: 2018.4.29f1, 2019.4.15f1, 2020.1.0a22
Could not test with: 2020.1.0a23, 2020.1.14f1, 2020.2.0a1, 2020.2.0b11, 2021.1.0a1, 2021.1.0a3(script does not invoke a progress bar at all)

Notes:
Script is borrowed from the DisplayProgressBar manual page of 2019.4 manual
With most progress bars(e.g. native Editor), the progress bar disappears for only a split second and then reappears when refocusing the Editor. However, for this particular script, the progress bar does not reappear unless the main Editor window is shaken around for a second or two when dragging