Docking windows that have a minimum size causes incorrect clipping rect

[Unit2Ed]

Version/Branch of Dear ImGui:

Version: 1.72 WIP (17102)
Branch: docking

Back-end/Renderer/Compiler/OS

Back-ends: imgui_impl_dx11.cpp + imgui_impl_win32.cpp
Compiler: Visual C++ 2017
Operating System: Windows 10

If you have a window that's set to have a minimum size, then dock the window and shrink it smaller than that minimum size, the content clip rect seems to be set to the minimum size, rather than the size enforced on the window by the dock node.

In the example project, I've created a simple window with a large button, and set a min/max constraint on it:

ImGui::SetNextWindowSizeConstraints(ImVec2(300, 300), ImVec2(800, 800));
ImGui::Begin("Constrained Window");
ImGui::Button("Button", ImVec2(600, 600));
ImGui::End();

Then used the demo window's Examples -> Dockspace menu to enable a full-screen dockspace, and dragged the window to the left edge, and shrunk the dock node's width to less than 300px. Notice that the button's text is visible outside the window, and the button's hover shading is too; clipping at around 300px from the edge of the screen.

I'm unsure whether there's a core problem that the dock node should respect the window's constraints and stop you from shrinking it so small, but it shouldn't be allowing the window to draw outside its bounds either way.

[ocornut]

Nice find, odd bug..
At the moment the Docking doesn't honor constraint. Ideally it would/should but that would require non-trivial work. Int he meanwhile I believe the suitable workaround would be explicitely disable them so they don't interfere:

in Begin() adding those two lines should fix it:

    if (first_begin_of_the_frame)
    {
        bool has_dock_node = (window->DockId != 0 || window->DockNode != NULL);
        bool new_auto_dock_node = !has_dock_node && GetWindowAlwaysWantOwnTabBar(window);
        if (has_dock_node || new_auto_dock_node)
        {
            BeginDocked(window, p_open);
            flags = window->Flags;

++          // Docking currently override constraints
++          g.NextWindowData.Flags &= ~ImGuiNextWindowDataFlags_HasSizeConstraint;
        }
    }

Would that be an acceptable fix short-term? (I don't have an ETA for docking handling constraint at window-level... it is kind of a hairy problem as windows with varied constraints may be docked in the same location.. toggling their visiblity would affect the docking tree every time..).

I think perhaps a healthier way of supporting them is that the constraints wouldn't affect the size of the docking node, but the space the window would be allowed to occupy within the space of that node would be affected by the constraint (so some space in the docking node may be unused).

[Unit2Ed]

Thanks Omar, I've confirmed that your patch has fixed this for us.
I agree, there's no sensible solution to size-constrained docking; we'll just need to make sure our windows do the best they can.