In D9830#189769, @dfaure wrote:

Nice investigation and fix! Do you happen to know the actual change in Qt which broke this? Wondering if it should 1) be mentioned in the commit log, 2) lead to an #if QT_VERSION.
But if it's just a supposition, then that's fine, don't spend a week tracking down the change in Qt :-)

I don't - there were some reports before Qt 5.10 (I never hit it myself though) already, so it's always been broken, just not this severe.
The change in Qt 5.10 is probably that some operation used by the preview generator calls processEvents or an eventLoop now instead of doing a fully-blocking operation - behaviour which kio definitely can't rely on.

I certainly hope QListView doesn't trigger a nested event loop, that would be horribly nasty and the cause for a million more problems.... (up to and including crashes when closing the view/window while this is happening). But I'm assuming this is just a supposition, and I'm strongly hoping it's unfounded ;-)

In D9830#189776, @dfaure wrote:

I certainly hope QListView doesn't trigger a nested event loop, that would be horribly nasty and the cause for a million more problems.... (up to and including crashes when closing the view/window while this is happening). But I'm assuming this is just a supposition, and I'm strongly hoping it's unfounded ;-)

Not QListView - the preview generator can do all kinds of stuff. There's also a slim change that something else does a full relayout - I didn't spot anything like that however.
Normally I'd expect QListView::setUniformItemSizes to trigger a relayout on change though, so this fix is in any case correct.