Using PopFont() inside a window causes crash when rendering text in that window

[codingoat]

Version: 1.81
Branch: master
Back-ends: imgui_impl_sdl.cpp + imgui_impl_opengl3.cpp
Operating System: Windows 10

My Issue/Question:

Hi! I am trying to set a custom font for my window titles (following the sample in this issue), and ran into a crash in the process.

I have loaded two fonts during initialization, fontSans and fontTitle. As expected, fontSans acts as the default font. However, if I try to set a window title font to fontTitle

ImGui::PushFont(fontTitle);
ImGui::Begin("Window");
ImGui::PopFont();
ImGui::Text("Hello!");
ImGui::End();

the program crashes with the following message: Assertion failed: font->ContainerAtlas->TexID == _CmdHeader.TextureId, file ...\imgui\imgui_draw.cpp, line 1402

Call stack:

 	ucrtbased.dll!00007ffee4687c75()	Unknown
 	ucrtbased.dll!00007ffee4687e13()	Unknown
 	ucrtbased.dll!00007ffee469e01d()	Unknown
 	ucrtbased.dll!00007ffee46a3c65()	Unknown
 	ucrtbased.dll!00007ffee46a37d7()	Unknown
 	ucrtbased.dll!00007ffee46a1868()	Unknown
 	ucrtbased.dll!00007ffee46a41cf()	Unknown
>	SpirMark.exe!ImDrawList::AddText(const ImFont * font, float font_size, const ImVec2 & pos, unsigned int col, const char * text_begin, const char * text_end, float wrap_width, const ImVec4 * cpu_fine_clip_rect) Line 1402	C++
 	SpirMark.exe!ImGui::RenderTextWrapped(ImVec2 pos, const char * text, const char * text_end, float wrap_width) Line 2663	C++
 	SpirMark.exe!ImGui::TextEx(const char * text, const char * text_end, int flags) Line 249	C++
 	SpirMark.exe!ImGui::TextV(const char * fmt, char * args) Line 273	C++
 	SpirMark.exe!ImGui::Text(const char * fmt, ...) Line 261	C++
 	SpirMark.exe!CMyApp::RenderUI() Line 347	C++
 	SpirMark.exe!CMyApp::Render() Line 312	C++
 	SpirMark.exe!SDL_main(int argc, char * * args) Line 212	C++
 	SpirMark.exe!main_utf8(int argc, char * * argv) Line 127	C
 	SpirMark.exe!main_getcmdline(...) Line 159	C
 	SpirMark.exe!main(int argc, char * * argv) Line 172	C

Meanwhile, doing this

ImGui::PushFont(fontTitle);
ImGui::Begin("Window");
ImGui::Text("Hello!");
ImGui::PopFont();
ImGui::End();

ImGui::Begin("Window 2");
ImGui::Text("Hello default!");
ImGui::End();

works, with the second window using the default font (as expected).

What am I doing wrong?

[ocornut]

Thank you for the report. I can confirm there is a bug with PopFont() leading to an empty stack.

[ocornut]

PushFont/PopFont are described as being "shared stack" but the way they behave actually isn't correctly behaving cross window.
So one way to see it is that it is a documentation bug (and PushFont/PopFont is not a shared stack)
But ideally I think everyone wants this to be a shared stack so I think it should be fixed toward honoring that behavior.

[ocornut]

Same as #3224 moving there.

[ocornut]

I don't mind to cop out from this but last time I looked at this - and been looking for two hours now - the right fix is surprisingly complex especially as they are other elements of the problem not mentioned in those issues: use of multiple atlas, etc.

May I suggest in the meanwhile that you workaround the problem using:

ImGui::PushFont(fontTitle);
ImGui::Begin("Window");
ImGui::PushFont(defaultFont);
ImGui::Text("Hello!");
ImGui::PopFont();
ImGui::End();
ImGui::PopFont();

[codingoat]

Using the workaround is alright with me, thank you for your help!

[ocornut]

FYI I have pushed a fix for this now: eb7201b