Fix BeginPopupContextWindow() to behave correctly when bound to a child

[jstine35]

I noticed while testing the Console sample in imgui-demo.cpp that the right-click context menu inside the child scroll-frame wouldn't show up sometimes (or most of the time, even). As best as I can determine, it would only work when no imgui window is active. Once any window is active -- even the parent/root of the child -- the right-click popup would no longer function.

[ocornut]

Thank you Jake, looking into it!
This was added in 695ea45 as part of #1382

Your patch fixes the console window (which has this problem because the text input field in the root window is force-activated at all time) but I think the issue is that we need to decide and clarify the default behavior of IsWindowHovered(). I added that test and support for the ImGuiHoveredFlags_AllowWhenBlockedByActiveItem flag primarily to be symmetrical with IsItemHovered() but looking into it now I am finding it hard to justify exactly how IsWindowHovered() should behave when there is an active item. My own codebase here doesn't use IsWindowHovered() very much.

Will look into it.
References, e.g. Lumix codebase:
https://github.com/nem0/LumixEngine/search?utf8=%E2%9C%93&q=IsWindowHovered&type=

[ocornut]

@jstine35 I have now pushed a fix for the issue, it's not your proposed PR as it follows logic that was already handled by IsItemHovered(), namely that we use the internal ActiveIdAllowOverlap flag.