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()
 {
     ImGuiContext& g = *GImGui;
-    g.CurrentWindow->DrawList->PopTextureID();
+    IM_ASSERT(g.FontStack.Size > 0);
     g.FontStack.pop_back();
-    SetCurrentFont(g.FontStack.empty() ? GetDefaultFont() : g.FontStack.back());
+    ImFont* font = g.FontStack.Size == 0 ? GetDefaultFont() : g.FontStack.back();
+    SetCurrentFont(font);
+    g.CurrentWindow->DrawList->_SetTextureID(font->ContainerAtlas->TexID);
 }
 
 void ImGui::PushItemFlag(ImGuiItemFlags option, bool enabled)

imgui.h

@@ -3152,6 +3152,7 @@ struct ImDrawList
     IMGUI_API void  _OnChangedClipRect();
     IMGUI_API void  _OnChangedTextureID();
     IMGUI_API void  _OnChangedVtxOffset();
+    IMGUI_API void  _SetTextureID(ImTextureID texture_id);
     IMGUI_API int   _CalcCircleAutoSegmentCount(float radius) const;
     IMGUI_API void  _PathArcToFastEx(const ImVec2& center, float radius, int a_min_sample, int a_max_sample, int a_step);
     IMGUI_API void  _PathArcToN(const ImVec2& center, float radius, float a_min, float a_max, int num_segments);

imgui_draw.cpp

@@ -526,7 +526,6 @@ void ImDrawList::_OnChangedClipRect()
         CmdBuffer.pop_back();
         return;
     }
-
     curr_cmd->ClipRect = _CmdHeader.ClipRect;
 }
 
@@ -549,7 +548,6 @@ void ImDrawList::_OnChangedTextureID()
         CmdBuffer.pop_back();
         return;
     }
-
     curr_cmd->TextureId = _CmdHeader.TextureId;
 }
 
@@ -625,6 +623,15 @@ void ImDrawList::PopTextureID()
     _OnChangedTextureID();
 }
 
+// This is used by ImGui::PushFont()/PopFont(). It works because we never use _TextureIdStack[] elsewhere than in PushTextureID()/PopTextureID().
+void ImDrawList::_SetTextureID(ImTextureID texture_id)
+{
+    if (_CmdHeader.TextureId == texture_id)
+        return;
+    _CmdHeader.TextureId = texture_id;
+    _OnChangedTextureID();
+}
+
 // Reserve space for a number of vertices and indices.
 // You must finish filling your reserved data before calling PrimReserve() again, as it may reallocate or
 // submit the intermediate results. PrimUnreserve() can be used to release unused allocations.