On OS X 10.11.2, when LR is running in full screen by clicking the green button in the upper-left corner of the main window, a resizable modal dialog box will automatically expand to full screen. This is clearly undesirable and doesn't happen on Windows.
Here's a sample script that shows the problem:
local LrDialogs = import 'LrDialogs'
local LrView = import 'LrView'
local f = LrView.osFactory()
LrDialogs.presentModalDialog {
title = "Full-Screen Bug", resizable = true,
contents = f:static_text {title = "Hello!"}}
[This also occurs with built-in dialogs such as File > Export... See below.]