Modal dialog remembering position

[pdoane]

I would like for a modal dialog to open in the center of the screen once, but then remember its location on subsequent uses. I tried adding this to the modal dialog demo:

            ImGui::SetNextWindowPosCenter(ImGuiSetCond_Once);

            if (ImGui::Button("Delete.."))
                ImGui::OpenPopup("Delete?");
            if (ImGui::BeginPopupModal("Delete?", NULL, ImGuiWindowFlags_AlwaysAutoResize))
            {
            }

But it still always opens in the center. Any thoughts or ideas?

[ocornut]

Implemented that change now.

Some very tangential thoughts (copying same message in #3091)

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Generally speaking, modals tends to be closer to regular windows than they are to popups, and I imagine a revamp where we could fully use Begin() with ImGuiWindowFlags_Modal (as suggested by #402) and keep control of visibility. The BeginPopupModal() api could still work.

Centering a modal window is surprisingly unobvious in a multi-viewport context, there are just many desirable scenarios. Considering an hypothetical GetFocusedViewport() returning ~~g.NavWindow->Viewport

SetNextWindowPos(GetFocusedViewport()->GetCenter(), ImGuiCond_Appearing, ImVec2(0.5f, 0.5f));

Won't work as expected due to how ImGuiCond_Appearing is set and because the focused/nav window tends to evolve across appearing frames. We probably need variety of access to monitors and viewports tagged with semantic (e.g. access to "focused (last interacted with?) monitor", access to "viewport from where popup was activated", access to "monitor mouse is in") and let the user choose. And I guess we may need to create more continuity between viewports and monitors down the line. Finishing work on the "there are no main viewport" task will probably help enforce this.

Currently the code and demo code uses io.DisplaySize * 0.5f in master branch and selectively CurrentWindow->Viewport's center or GetMainViewport()'s center in docking branch, which are ALL arbitrary, we ought to design standards and have them integrated in master asap.
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