Reproduction steps:
1. Open any project with Unity editor
2. Switch the platform to iOS
3. Go File -> Build Settings and start the build for iOS
4. From the moment the build progress window appears, try moving other OS windows on top of it (e.g. Finder)
5. Focus Unity and repeat steps 4 and 5 until the build completes

Expected result: the progress bar window will not be visible once other windows are on top of it
Actual result: the window is visible during some build phases even when other windows on top of it are selected

6. If the issue did not reproduce, try building again on top of the same folder using replace option
7. Repeat steps 4, 5 until the build completes

Reproducible with: 2018.4.0f1, 2018.4.34f1, 2019.4.24f1, 2020.3.5f1, 20201.1.4f1, 2021.2.0a14

Not reproducible with Windows, Linux

Notes:
1. A video is attached below that shows the problem 2021-04-20 10-49-00.mp4 (3.5 MB)
2. This issue happens on other platforms as well, but the iOS build has more build phases where the issue reproduces
3. This always seems to happen at the first phase of any platform build
4. Same issue can happen with Build Settings window