I tried this - backdrop "on" - on my Antergos/KDE computer and in the VM running Fedora 28/KDE (the Fedora/KDE spin). It mostly works. It seems that the tools palette still can be made to disappear when opening a property inspector but once the tools palette is brought back, things seem to behave normally. It does seem to eliminate the interaction between the message box and the tools palette. It is definitely an improvement. Thanks again for all your experimenting and for sharing the results!
The two VMs I have set up running Xfce aren't responding as well to this trick. The distros are Antergos (Arch) and Fedora 28. This Fedora is the spin version specifically dedicated to Xfce, not the main Workstation with Xfce added. The message box in both cases is all but unusable. It refuses to accept proper focus and keyboard input. The backdrop has no balming effect here in Xfce. I have discovered that opening a file, for example a text file, in a non-LiveCode app, while in the same workspace, allows the message box to accept focus and keyboard input. Until it's moved or some other part of LiveCode's interface is given focus. Also leaving and returning to the workspace brings the problem back
Deploying to Linux? Get penguinated here.