I have Ubuntu 17.10 with GNOME 3.
I have (for example) LibreOffice Calc open, with some data columns in it. I highlight the columns I want to chart, then I select INSERT > CHART. The chart child window dialog pops up on top of my data, so I want to move it to the side.
When I grab the top bar and move it, the parent (spreadsheet) window minimizes (I think it actually disappears). Continuing the above example, if I click FINISH on the chart window, I can then see the minimized spreadsheet application, which is the window title bar plus the menu bar just below it.
This seems like really odd behaviour and certainly falls under the category of "not being useful to the user", since the whole idea of moving the child window in the first place was just to be able to better see the spreadsheet!
In other applications, for example the PDF viewer, a slightly different problem occurs. When I try to move the child window (for example the print dialog) the parent does not minimize but instead follows the child window around. This is less surprising than having the parent disappear entirely but equally useless in terms of allowing the user to see the information present in the parent window.
Pomsky's answer below fixes the problem, but leaves me wondering why the window manager is making the decision that all child windows have modal behaviour (or don't), and not the applications themselves; and further, why modal behaviour prevents moving the child screen around when it obscures useful info on the parent screen. Neither of these seem like good design to me...