Suppose I've expanded a font family selection combo-box on a toolbar, and then clicked in the combo box' main area (to start typing something). Should the expanded list now be collapsed, or shouldn't it? I claim, that it shouldn't, and that, instead, the typing should either search the list box (like in the font family selection textbox+listbox combination in the Font dialog pane), or filter the list box (as suggested for the textbox+listbox combination in the recently-filed bug 156108).
If you use something like gtk3-demo to explore how gtk widgets work and launch the "Combo Boxes" example, then the GtkComboBoxText example there works (unsurprisingly) like it does in LibreOffice in that a click in the text area dismisses an active popup. Popups are often quite gruesome on many platforms and it can be quite technically challenging to do anything else given the complexity of grabs and how events are managed when there is a popup active. It's fairly strongly baked in that clicks outside a popup dismiss active popups.
(In reply to Caolán McNamara from comment #1) 1. A request for enhancement can't be resolved as NOTABUG, since it's not a bug in the first place... it can be INVALID or WONTFIX. 2. You seem to be saying that it would be difficult/not worth the effort to do this. But you have not argued against the utility of being able to search the list by partial typing. So, is it not the case that this issue should remain open, but with your comment that working on it would be taxing? (not reopening for now since I let this be closed for a while before replying; but will reopen as UNCONFIRMED given no further input.)