CheckboxFlags: Display mixed-value/tristate marker when passed flags that have multiple bits set and stored value matches neither zero neither the full set.

docs/CHANGELOG.txt

@@ -80,6 +80,8 @@ Other Changes:
   with signed and unsigned types. Added reverse Sliders to Demo. (#3432, #3449) [@rokups]
 - Text: Bypass unnecessary formatting when using the TextColored()/TextWrapped()/TextDisabled() helpers
   with a "%s" format string. (#3466)
+- CheckboxFlags: Display mixed-value/tristate marker when passed flags that have multiple bits set and
+  stored value matches neither zero neither the full set.
 - BeginMenuBar: Fixed minor bug where CursorPosMax gets pushed to CursorPos prior to calling BeginMenuBar(),
   so e.g. calling the function at the end of a window would often add +ItemSpacing.y to scrolling range.
 - TreeNode, CollapsingHeader: Made clicking on arrow toggle toggle the open state on the Mouse Down event

imgui_widgets.cpp

@@ -1071,7 +1071,8 @@ bool ImGui::Checkbox(const char* label, bool* v)
     RenderNavHighlight(total_bb, id);
     RenderFrame(check_bb.Min, check_bb.Max, GetColorU32((held && hovered) ? ImGuiCol_FrameBgActive : hovered ? ImGuiCol_FrameBgHovered : ImGuiCol_FrameBg), true, style.FrameRounding);
     ImU32 check_col = GetColorU32(ImGuiCol_CheckMark);
-    if (window->DC.ItemFlags & ImGuiItemFlags_MixedValue)
+    bool mixed_value = (window->DC.ItemFlags & ImGuiItemFlags_MixedValue) != 0;
+    if (mixed_value)
     {
         // Undocumented tristate/mixed/indeterminate checkbox (#2644)
         ImVec2 pad(ImMax(1.0f, IM_FLOOR(square_sz / 3.6f)), ImMax(1.0f, IM_FLOOR(square_sz / 3.6f)));
@@ -1084,7 +1085,7 @@ bool ImGui::Checkbox(const char* label, bool* v)
     }
 
     if (g.LogEnabled)
-        LogRenderedText(&total_bb.Min, *v ? "[x]" : "[ ]");
+        LogRenderedText(&total_bb.Min, mixed_value ? "[~]" : *v ? "[x]" : "[ ]");
     if (label_size.x > 0.0f)
         RenderText(ImVec2(check_bb.Max.x + style.ItemInnerSpacing.x, check_bb.Min.y + style.FramePadding.y), label);
 
@@ -1095,7 +1096,21 @@ bool ImGui::Checkbox(const char* label, bool* v)
 bool ImGui::CheckboxFlags(const char* label, unsigned int* flags, unsigned int flags_value)
 {
     bool v = ((*flags & flags_value) == flags_value);
-    bool pressed = Checkbox(label, &v);
+    bool pressed;
+    if (v == false && (*flags & flags_value) != 0)
+    {
+        // Mixed value (FIXME: find a way to expose neatly to Checkbox?)
+        ImGuiWindow* window = GetCurrentWindow();
+        const ImGuiItemFlags backup_item_flags = window->DC.ItemFlags;
+        window->DC.ItemFlags |= ImGuiItemFlags_MixedValue;
+        pressed = Checkbox(label, &v);
+        window->DC.ItemFlags = backup_item_flags;
+    }
+    else
+    {
+        // Regular checkbox
+        pressed = Checkbox(label, &v);
+    }
     if (pressed)
     {
         if (v)