Combo: added ImGuiComboFlags_WidthFitPreview. (#6881)

imgui.h

@@ -1115,6 +1115,7 @@ enum ImGuiComboFlags_
     ImGuiComboFlags_HeightLargest           = 1 << 4,   // As many fitting items as possible
     ImGuiComboFlags_NoArrowButton           = 1 << 5,   // Display on the preview box without the square arrow button
     ImGuiComboFlags_NoPreview               = 1 << 6,   // Display only a square arrow button
+    ImGuiComboFlags_WidthFitPreview         = 1 << 7,   // Dynamic width depending on current selected element
     ImGuiComboFlags_HeightMask_             = ImGuiComboFlags_HeightSmall | ImGuiComboFlags_HeightRegular | ImGuiComboFlags_HeightLarge | ImGuiComboFlags_HeightLargest,
 };
 

imgui_demo.cpp

@@ -1190,6 +1190,7 @@ static void ShowDemoWindowWidgets()
         static ImGuiComboFlags flags = 0;
         ImGui::CheckboxFlags("ImGuiComboFlags_PopupAlignLeft", &flags, ImGuiComboFlags_PopupAlignLeft);
         ImGui::SameLine(); HelpMarker("Only makes a difference if the popup is larger than the combo");
+        ImGui::CheckboxFlags("ImGuiComboFlags_WidthFitPreview", &flags, ImGuiComboFlags_WidthFitPreview);
         if (ImGui::CheckboxFlags("ImGuiComboFlags_NoArrowButton", &flags, ImGuiComboFlags_NoArrowButton))
             flags &= ~ImGuiComboFlags_NoPreview;     // Clear the other flag, as we cannot combine both
         if (ImGui::CheckboxFlags("ImGuiComboFlags_NoPreview", &flags, ImGuiComboFlags_NoPreview))

imgui_widgets.cpp

@@ -1686,7 +1686,8 @@ bool ImGui::BeginCombo(const char* label, const char* preview_value, ImGuiComboF
 
     const float arrow_size = (flags & ImGuiComboFlags_NoArrowButton) ? 0.0f : GetFrameHeight();
     const ImVec2 label_size = CalcTextSize(label, NULL, true);
-    const float w = (flags & ImGuiComboFlags_NoPreview) ? arrow_size : CalcItemWidth();
+    const float preview_width = (preview_value != nullptr) ? CalcTextSize(preview_value, NULL, true).x : 0.0f;
+    const float w = (flags & ImGuiComboFlags_NoPreview) ? arrow_size : ((flags & ImGuiComboFlags_WidthFitPreview) ? arrow_size + preview_width + style.ItemInnerSpacing.x * 2.0f : CalcItemWidth());
     const ImRect bb(window->DC.CursorPos, window->DC.CursorPos + ImVec2(w, label_size.y + style.FramePadding.y * 2.0f));
     const ImRect total_bb(bb.Min, bb.Max + ImVec2(label_size.x > 0.0f ? style.ItemInnerSpacing.x + label_size.x : 0.0f, 0.0f));
     ItemSize(total_bb, style.FramePadding.y);