Two childs not overlapping eachother

[AbbyIOM]

Thank you!

My Issue/Question:

I've got two childs, one child(1) has a button/text init the second child(2) has a move button that moves the child(2) on top of the other child(1), my problem is that even though the child() is on top of the child(1) the button/text is still visible.

Screenshots/Video

2021-08-27.22-36-14.mp4

// Here's some code anyone can copy and paste to reproduce your issue
            ImGui::SetNextWindowPos(ImVec2(100, 200));
            ImGui::BeginChild("Child #1", ImVec2(200, 200), true);
            ImGui::Text("Child #1 Text");
            ImGui::EndChild();


            ImGui::SetNextWindowPos(ImVec2(movePos, 200));
            ImGui::BeginChild("Child #2", ImVec2(200, 200), true);
            ImGui::SetCursorPosY(50);
            if (ImGui::Button("Move Child #2 on Child #1"))
                move = !move;
            ImGui::EndChild();

[ocornut]

The ChildBg rectangle is drawn on the parent to save a draw call. If you look for that code drawing the background you’ll find there’s a rule for it you may be able to workaround.

Check the code that calls RenderWindowDecorations().

[ocornut]

Detecting overlapping childs would be a O(log N) operation I don’t fancy but since our heuristic lastest so long (since 1.71) without another report, I think we could add to checking of previous child as part of the heuristic and it would fix a good portionncases (including yours which technically is the first reported case).

[AbbyIOM]

The ChildBg rectangle is drawn on the parent to save a draw call. If you look for that code drawing the background you’ll find there’s a rule for it you may be able to workaround.

Check the code that calls RenderWindowDecorations().

Thank you for the fast reply! Will do.

[ocornut]

Pushed a fix for this case.

Here's some minimal working test:

ImGui::Text("Hello from parent");
ImVec2 p = ImGui::GetCursorScreenPos();
ImGui::PushStyleColor(ImGuiCol_ChildBg, ImVec4(0.5f, 0.0f, 0.0f, 1.0f));
ImGui::SetNextWindowPos(ImVec2(p.x+100, p.y+0));
ImGui::BeginChild("Child #1", ImVec2(200, 200), true);
ImGui::Text("Child #1 Text");
ImGui::Button("Child #1 long button");
ImGui::EndChild();
ImGui::PopStyleColor();

ImGui::PushStyleColor(ImGuiCol_ChildBg, ImVec4(0.0f, 0.5f, 0.0f, 1.0f));
ImGui::SetNextWindowPos(ImVec2(p.x+150, p.y+0));
ImGui::BeginChild("Child #2", ImVec2(200, 200), true);
ImGui::SetCursorPosY(50);
ImGui::Button("Some button");
ImGui::EndChild();
ImGui::PopStyleColor();

[cfillion]

// Child windows can render their decoration (bg color, border, scrollbars, etc.) within their parent to save a draw call (since 1.71)
// FIXME: [...] Please get in contact if you are affected (github #4493)

This optimization causes z-order issues when a child window overlaps another submitted earlier, before the previous.

Minimal code snippet to duplicate

if(ImGui::BeginChildFrame(ImGui::GetID("under1"), { -FLT_MIN, 28 })) {
  ImGui::Text("aaaaaaaaaaaaaaaaa");
  ImGui::EndChildFrame();
}
if(ImGui::BeginChildFrame(ImGui::GetID("under2"), { -FLT_MIN, 28 })) {
  ImGui::Text("bbbbbbbbbbbbbbbbb");
  ImGui::EndChildFrame();
}

ImGui::PushStyleColor(ImGuiCol_ChildBg, IM_COL32(0, 0, 0, 0xcc));
ImGui::PushStyleColor(ImGuiCol_Text, IM_COL32(0xff, 0, 0xff, 0xff));
// overlaps under1 but not under2
ImGui::SetCursorPos({ 10, 32 });
if(ImGui::BeginChild("over1", { 50, 20 })) {
  ImGui::Text("over");
  ImGui::EndChild();
}
// overlaps both under1 and under2 but not over1
ImGui::SetCursorPos({ 70, 32 });
if(ImGui::BeginChild("over2", { 50, 50 })) {
  ImGui::Text("over");
  ImGui::EndChild();
}
ImGui::PopStyleColor(2);

An example of the problem occurring in a real-world scenario (script by @GoranKovac): user-positioned child windows (InputTextMultiline) are dynamically submitted and any of them may underlap later child windows (the toolbars):

[ocornut]

This optimization is quite meaningful, so considering we haven't been getting too many requests for that and considering your specific use case, I would suggest to aim for a workaround.

• You may submit child without decoration and draw background and border yourself? Considering the application here it seems like this would be trivial? May need to pass ImGuiWindowFlags_AlwaysUseWindowPadding to child.
• Or you may trick the heuristic by leaving the parent empty of vertices (e.g adding an intermediate invisible child window to hold all those).

[ocornut]

If you can think of a way to amend the existing heuristic I'm also open to that.

(Btw there was a minor typo in the code parent_is_empty was parent_is_not_empty, I reworked the condition accordingly so the name is not misleading anymore. That'll be in an upcoming commit but it has no effect on compiled code)