Issue programmatically setting Scrolling with SetScrollHereY() in window with clipped contents

[llxiaoyuan]

Version/Branch of Dear ImGui:

Version 1.XX, Branch: XXX (master/docking/etc.)

Back-ends:

imgui_impl_XXX.cpp + imgui_impl_XXX.cpp

Compiler, OS:

win10

Full config/build information:

No response

Details:

why I need to click twice to get the scroll bar at right pos

Screenshots/Video:

No response

Minimal, Complete and Verifiable Example code:

        static bool need_to_set_scroll = false;
        static bool need_to_set_scroll2 = false;
        static bool need_to_set_scroll3 = false;
        ImGui::Begin(u8"chat111");
        if (ImGui::Button("track_once111"))
        {
            need_to_set_scroll = true;
            need_to_set_scroll2 = true;
            need_to_set_scroll3 = true;
        }
        ImGui::BeginChild("chatBeginChild", ImVec2(280, 280), ImGuiChildFlags_Border, ImGuiWindowFlags_HorizontalScrollbar);

        ImGui::BeginChild("chatBeginChild_1", ImVec2(300, 300), ImGuiChildFlags_Border);
        int scroll_index = 49;
        for (size_t j = 0; j < 50; j++) {
            ImGui::TextColored(ImVec4(1, 1, 1, 1), "%d %d", j, scroll_index);
            if (need_to_set_scroll && scroll_index == j) {
                need_to_set_scroll = false;
                printf("SetScrollHereY called\n");
                ImGui::SetScrollHereY(0.5f);
            }
        }
        ImGui::EndChild();

        ImGui::SameLine();

        ImGui::BeginChild("chatBeginChild_2", ImVec2(300, 300), ImGuiChildFlags_Border);
        int scroll_index2 = 49;
        for (size_t j = 0; j < 50; j++) {
            ImGui::TextColored(ImVec4(1, 1, 1, 1), "%d %d", j, scroll_index2);
            if (need_to_set_scroll2 && scroll_index2 == j) {
                need_to_set_scroll2 = false;
                printf("SetScrollHereY2 called\n");
                ImGui::SetScrollHereY(0.5f);
            }
        }
        ImGui::EndChild();

        if (need_to_set_scroll3) {
            need_to_set_scroll3 = false;
            printf("SetScrollHereX called\n");
            printf("\n");
            ImGui::SetScrollHereX(0.5f);
        }

        ImGui::EndChild();
        ImGui::End();

[llxiaoyuan]

@ocornut It seems that setting y is invalid when the column is not displayed

[llxiaoyuan]

I can solve some problems in this way, But I don't think that's a good idea

if (ImGui::IsRectVisible(ImGui::GetContentRegionAvail())){
  drawCol();
}

[ocornut]

The problem you have is that BeginChild() returns false when not visible and goes in a state when none of the element submitted are processed (internally it sets window->SkipItems = true). So when the child is 100% not visible, not of the Text() calls are going through and SetScrollHere() functions are still referring to the start position.

Possible solutions:
(A) either you queue/store your request to be processed when the window is visible.

if (ImGui::BeginChild("chatBeginChild_2", ImVec2(300, 300), ImGuiChildFlags_Borders))
{
    int scroll_index2 = 49;
    for (size_t j = 0; j < 50; j++)
    {
        ImGui::TextColored(ImVec4(1, 1, 1, 1), "%d %d", j, scroll_index2);
        if (need_to_set_scroll2 && scroll_index2 == j)
        {
            need_to_set_scroll2 = false;
            IMGUI_DEBUG_LOG("SetScrollHereY2 called\n");
            ImGui::SetScrollHereY(0.5f);
        }
    }
}
ImGui::EndChild();

And need_to_set_scroll2 needs to persist across frames until used.

(B) Either you compute the scroll target rather than rely on current layout:

ImGui::BeginChild("chatBeginChild_2", ImVec2(300, 300), ImGuiChildFlags_Borders);
int scroll_index2 = 49;
if (need_to_set_scroll2)
{
    ImGui::SetScrollY(ImGui::GetTextLineHeightWithSpacing() * scroll_index2);
    need_to_set_scroll2 = false;
}
for (size_t j = 0; j < 50; j++)
    ImGui::TextColored(ImVec4(1, 1, 1, 1), "%d %d", j, scroll_index2);
ImGui::EndChild();