Fonts: Made it possible to use PushFont()/PopFont() calls accross Begin() calls. (#3224, #3875, #6398, #7903)

docs/CHANGELOG.txt

@@ -51,6 +51,7 @@ Other changes:
 - MultiSelect+TreeNode: default open behavior is OpenOnDoubleClick + OpenOnArrow
   when used in a multi-select context without any OpenOnXXX flags set. (#7850)
 - TextLink(), TextLinkOpenURL(): change mouse cursor to Hand shape when hovered. (#7885, #7660)
+- Fonts: Made it possible to use PushFont()/PopFont() calls accross Begin() calls. (#3224, #3875, #6398, #7903)
 - Backends: GLFW: added ImGui_ImplGlfw_Sleep() helper function because GLFW does not
   provide a way to do a portable sleep. (#7844)
 - Backends: SDL2, SDL3: ignore events of other SDL windows. (#7853) [@madebr, @ocornut]

imgui.cpp

@@ -5068,6 +5068,8 @@ static void InitViewportDrawData(ImGuiViewportP* viewport)
 // - If the code here changes, may need to update code of functions like NextColumn() and PushColumnClipRect():
 //   some frequently called functions which to modify both channels and clipping simultaneously tend to use the
 //   more specialized SetWindowClipRectBeforeSetChannel() to avoid extraneous updates of underlying ImDrawCmds.
+// - This is analoguous to PushFont()/PopFont() in the sense that are a mixing a global stack and a window stack,
+//   which in the case of ClipRect is not so problematic but tends to be more restrictive for fonts.
 void ImGui::PushClipRect(const ImVec2& clip_rect_min, const ImVec2& clip_rect_max, bool intersect_with_current_clip_rect)
 {
     ImGuiWindow* window = GetCurrentWindow();
@@ -7574,22 +7576,31 @@ void ImGui::SetCurrentFont(ImFont* font)
     g.DrawListSharedData.FontScale = g.FontScale;
 }
 
+// Use ImDrawList::_SetTextureID(), making our shared g.FontStack[] authorative against window-local ImDrawList.
+// - Whereas ImDrawList::PushTextureID()/PopTextureID() is not to be used across Begin() calls.
+// - Note that we don't propagate current texture id when e.g. Begin()-ing into a new window, we never really did...
+//   - Some code paths never really fully worked with multiple atlas textures.
+//   - The right-ish solution may be to remove _SetTextureID() and make AddText/RenderText lazily call PushTextureID()/PopTextureID()
+//     the same way AddImage() does, but then all other primitives would also need to? I don't think we should tackle this problem
+//     because we have a concrete need and a test bed for multiple atlas textures.
 void ImGui::PushFont(ImFont* font)
 {
     ImGuiContext& g = *GImGui;
-    if (!font)
+    if (font == NULL)
         font = GetDefaultFont();
-    SetCurrentFont(font);
     g.FontStack.push_back(font);
-    g.CurrentWindow->DrawList->PushTextureID(font->ContainerAtlas->TexID);
+    SetCurrentFont(font);
+    g.CurrentWindow->DrawList->_SetTextureID(font->ContainerAtlas->TexID);
 }
 
 void  ImGui::PopFont()<