TextWrapped causes "Too many vertices in ImDrawList" even with VfxOffset

[dbr]

Version/Branch of Dear ImGui:

Version: 1.86
Branch: master

Back-end/Renderer/Compiler/OS

Back-ends: Via imgui-rs, using imgui-glow-renderer

My Issue/Question:

As per imgui-rs/imgui-rs#331 (comment)

Displaying a long (1000 lines with 100 characters per line) string in TextWrapped triggers this assertion:

hello_world: ./third-party/imgui-master/imgui/imgui.cpp:4364: void AddDrawListToDrawData(ImVector<ImDrawList*>*, ImDrawList*): Assertion `draw_list->_VtxCurrentIdx < (1 << 16) && "Too many vertices in ImDrawList using 16-bit indices. Read comment above"' failed.

This is with a backend that has VfxOffset support - I think all the text is being added in a single DrawCmd thus cannot use the VfxOffset

Screenshots/Video

Standalone, minimal, complete and verifiable example:

Following can be put in imgui-examples/examples/something.rs in https://github.com/imgui-rs/imgui-rs/ and run via cargo run --example something - but should be pretty trivial to convert into C++!

use imgui::*;

mod support;

fn main() {
    let system = support::init(file!());
    let mut text = String::new();
    for _row in 0..1_000 {
        for _char in 0..100 {
            text += "a";
        }
        text += "\n";
    }
    system.main_loop(move |_, ui| {
        ui.window("Hello world")
            .size([300.0, 110.0], Condition::FirstUseEver)
            .build(|| {
                ui.child_window("Hm").build(||{
                    ui.text_wrapped(&text);
                });
            });
    });
}

[ocornut]

The code near the beginning of ImFont::RenderText() does two things for which word-wrapping is not supported:

• // Fast-forward to first visible line
• // For large text, scan for the last visible line in order to avoid over-reserving in the call to PrimReserve()

In this instance adding support for word-wrapping in the first thing (the fast-forward) would be fairly straightforward and fix your issue.

[ocornut]

Pushed a fix in two steps:

• Small refactor 4d4889b
• Added fast-forward clipping when wrapped e0330c1

The algorithm is not optimal due to specs of CalcWordWrapPositionA() but it is still way faster than with the previous code, and also obviously fixes thing.

Using debug view and mapping the new code on a keyboard modifier (e.g. && ImGui::GetIO().KeyShift test) we can confirm that vertices are not emitted anymore above the clipping line:

Repro code:

static ImGuiTextBuffer text_buf;
if (text_buf.empty())
   for (int line = 0; line < 500; line++)
        text_buf.append("aaaaaaaaaabbbbbbbbbbccccccccccddddddddddeeeeeeeeeeaaaaaaaaaabbbbbbbbbbccccccccccddddddddddeeeeeeeeee\n");
ImGui::TextWrapped("%s", text_buf.c_str());