Tables: Fix calculation of auto-fit (remove padding). Demo setting a width in columns setup + ImGuiTableFlags_NoKeepColumnsVisible.

imgui.h

@@ -1041,7 +1041,7 @@ enum ImGuiTableFlags_
     ImGuiTableFlags_SizingPolicyStretchX            = 1 << 14,  // Default if ScrollX is off. Columns will default to use _WidthStretch policy. Read description above for more details.
     ImGuiTableFlags_NoHeadersWidth                  = 1 << 15,  // Disable header width contribution to automatic width calculation.
     ImGuiTableFlags_NoHostExtendY                   = 1 << 16,  // (FIXME-TABLE: Reword as SizingPolicy?) Disable extending past the limit set by outer_size.y, only meaningful when neither of ScrollX|ScrollY are set (data below the limit will be clipped and not visible)
-    ImGuiTableFlags_NoKeepColumnsVisible            = 1 << 17,  // (FIXME-TABLE) Disable code that keeps column always minimally visible when table width gets too small.
+    ImGuiTableFlags_NoKeepColumnsVisible            = 1 << 17,  // (FIXME-TABLE) Disable code that keeps column always minimally visible when table width gets too small and horizontal scrolling is off.
     // Scrolling
     ImGuiTableFlags_ScrollX                         = 1 << 18,  // Enable horizontal scrolling. Require 'outer_size' parameter of BeginTable() to specify the container size. Because this create a child window, ScrollY is currently generally recommended when using ScrollX.
     ImGuiTableFlags_ScrollY                         = 1 << 19,  // Enable vertical scrolling. Require 'outer_size' parameter of BeginTable() to specify the container size.

imgui_demo.cpp

@@ -3596,6 +3596,32 @@ static void ShowDemoWindowTables()
         ImGui::TreePop();
     }
 
+    if (open_action != -1)
+        ImGui::SetNextItemOpen(open_action != 0);
+    if (ImGui::TreeNode("Explicit widths"))
+    {
+        static ImGuiTableFlags flags = ImGuiTableFlags_None;
+        ImGui::CheckboxFlags("ImGuiTableFlags_NoKeepColumnsVisible", (unsigned int*)&flags, ImGuiTableFlags_NoKeepColumnsVisible);
+        if (ImGui::BeginTable("##table1", 3, flags))
+        {
+            // We could also set ImGuiTableFlags_SizingPolicyFixedX on the table and all columns will default to ImGuiTableColumnFlags_WidthFixed.
+            ImGui::TableSetupColumn("", ImGuiTableColumnFlags_WidthFixed, 100.0f);
+            ImGui::TableSetupColumn("", ImGuiTableColumnFlags_WidthFixed, 200.0f);
+            ImGui::TableSetupColumn("", ImGuiTableColumnFlags_WidthFixed, 100.0f);
+            for (int row = 0; row < 5; row++)
+            {
+                ImGui::TableNextRow();
+                for (int column = 0; column < 3; column++)
+                {
+                    ImGui::TableSetColumnIndex(column);
+                    ImGui::Text("Hello %d,%d", row, column);
+                }
+            }
+            ImGui::EndTable();
+        }
+        ImGui::TreePop();
+    }
+
     if (open_action != -1)
         ImGui::SetNextItemOpen(open_action != 0);
     if (ImGui::TreeNode("Vertical scrolling, with clipping"))
@@ -4223,6 +4249,8 @@ static void ShowDemoWindowTables()
             ImGui::SameLine(); HelpMarker("[Default if ScrollX is on]\nEnlarge as needed: enable scrollbar if ScrollX is enabled, otherwise extend parent window's contents rectangle. Only Fixed columns allowed. Stretched columns will calculate their width assuming no scrolling.");
             ImGui::CheckboxFlags("ImGuiTableFlags_NoHeadersWidth", (unsigned int*)&flags, ImGuiTableFlags_NoHeadersWidth);
             ImGui::CheckboxFlags("ImGuiTableFlags_NoHostExtendY", (unsigned int*)&flags, ImGuiTableFlags_NoHostExtendY);
+            ImGui::CheckboxFlags("ImGuiTableFlags_NoKeepColumnsVisible", (unsigned int*)&flags, ImGuiTableFlags_NoKeepColumnsVisible);
+            ImGui::SameLine(); HelpMarker("Only available if ScrollX is disabled.");
             ImGui::Unindent();
 
             ImGui::BulletText("Scrolling:");

imgui_tables.cpp

@@ -580,7 +580,6 @@ void    ImGui::TableUpdateLayout(ImGuiTable* table)
     // (can't make auto padding larger than what WorkRect knows about so right-alignment matches)
     const ImRect work_rect = table->WorkRect;
     const float padding_auto_x = table->CellPaddingX2;
-    const float spacing_auto_x = table->CellSpacingX * (1.0f + 2.0f); // CellSpacingX is >0.0f when there's no vertical border, in which case we add two extra CellSpacingX to make auto-fit look nice instead of cramped. We may want to expose this somehow.
     const float min_column_width = TableGetMinColumnWidth();
 
     int count_fixed = 0;
@@ -614,7 +613,11 @@ void    ImGui::TableUpdateLayout(ImGuiTable* table)
         if (!(table->Flags & ImGuiTableFlags_NoHeadersWidth) && !(column->Flags & ImGuiTableColumnFlags_NoHeaderWidth))
             column_width_ideal = ImMax(column_width_ideal, column_content_width_headers);
         column_width_ideal = ImMax(column_width_ideal + padding_auto_x, min_column_width);
+
+        // CellSpacingX is >0.0f when there's no vertical border
         table->ColumnsAutoFitWidth += column_width_ideal;
+        if (column->PrevVisibleColumn != -1)
+            table->ColumnsAutoFitWidth += table->CellSpacingX;
 
         if (column->Flags & (ImGuiTableColumnFlags_WidthAlwaysAutoResize | ImGuiTableColumnFlags_WidthFixed))
         {
@@ -647,10 +650,6 @@ void    ImGui::TableUpdateLayout(ImGuiTable* table)
         }
     }
 
-    // CellSpacingX is >0.0f when there's no vertical border, in which case we add two extra CellSpacingX to make auto-fit look nice instead of cramped.
-    // We may want to expose this somehow.
-    table->ColumnsAutoFitWidth += spacing_auto_x * (table->ColumnsVisibleCount - 1);
-
     // Layout
     const float width_spacings = table->CellSpacingX * (table->ColumnsVisibleCount - 1);
     float width_avail;