Tables: fixed matching width of synchronized tables when only some (not all) instances have a vertical scrollbar. (#5920)

docs/CHANGELOG.txt

@@ -53,6 +53,8 @@ Other changes:
   which should later lead us toward more flexible uses of menu/status bars. (#5143, #3692)
 - Tables, Nav: frozen columns are not part of menu layer and can be crossed over. (#5143, #3692)
 - Tables, Columns: fixed cases where empty columns may lead to empty ImDrawCmd. (#4857, #5937)
+- Tables: fixed matching width of synchronized tables (multiple tables with same id) when only
+  some instances have a vertical scrollbar and not all. (#5920)
 - Inputs, IO: reworked ImGuiMod_Shortcut to redirect to Ctrl/Super at runtime instead of
   compile-time, being consistent with our support for io.ConfigMacOSXBehaviors and making it
   easier for bindings generators to process that value. (#5923, #456)

imgui_demo.cpp

@@ -5043,18 +5043,23 @@ static void ShowDemoWindowTables()
     if (ImGui::TreeNode("Synced instances"))
     {
         HelpMarker("Multiple tables with the same identifier will share their settings, width, visibility, order etc.");
+
+        static ImGuiTableFlags flags = ImGuiTableFlags_Resizable | ImGuiTableFlags_Reorderable | ImGuiTableFlags_Hideable | ImGuiTableFlags_Borders | ImGuiTableFlags_SizingFixedFit | ImGuiTableFlags_NoSavedSettings;
+        ImGui::CheckboxFlags("ImGuiTableFlags_ScrollY", &flags, ImGuiTableFlags_ScrollY);
+        ImGui::CheckboxFlags("ImGuiTableFlags_SizingFixedFit", &flags, ImGuiTableFlags_SizingFixedFit);
         for (int n = 0; n < 3; n++)
         {
             char buf[32];
             sprintf(buf, "Synced Table %d", n);
             bool open = ImGui::CollapsingHeader(buf, ImGuiTreeNodeFlags_DefaultOpen);
-            if (open && ImGui::BeginTable("Table", 3, ImGuiTableFlags_Resizable | ImGuiTableFlags_Reorderable | ImGuiTableFlags_Hideable | ImGuiTableFlags_Borders | ImGuiTableFlags_SizingFixedFit | ImGuiTableFlags_NoSavedSettings))
+            if (open && ImGui::BeginTable("Table", 3, flags, ImVec2(0.0f, ImGui::GetTextLineHeightWithSpacing() * 5)))
             {
                 ImGui::TableSetupColumn("One");
                 ImGui::TableSetupColumn("Two");
                 ImGui::TableSetupColumn("Three");
                 ImGui::TableHeadersRow();
-                for (int cell = 0; cell < 9; cell++)
+                const int cell_count = (n == 1) ? 27 : 9; // Make second table have a scrollbar to verify that additional decoration is not affecting column positions.
+                for (int cell = 0; cell < cell_count; cell++)
                 {
                     ImGui::TableNextColumn();
                     ImGui::Text("this cell %d", cell);

imgui_internal.h

@@ -2507,7 +2507,7 @@ struct ImGuiTableInstanceData
     ImGuiTableInstanceData()    { LastOuterHeight = LastFirstRowHeight = LastFrozenHeight = 0.0f; }
 };
 
-// FIXME-TABLE: more transient data could be stored in a per-stacked table structure: DrawSplitter, SortSpecs, incoming RowData
+// FIXME-TABLE: more transient data could be stored in a stacked ImGuiTableTempData: e.g. SortSpecs, incoming RowData
 struct IMGUI_API ImGuiTable
 {
     ImGuiID                     ID;
@@ -2612,6 +2612,8 @@ struct IMGUI_API ImGuiTable
     bool                        IsResetDisplayOrderRequest;
     bool                        IsUnfrozenRows;             // Set when we got past the frozen row.
     bool                        IsDefaultSizingPolicy;      // Set if user didn't explicitly set a sizing policy in BeginTable()
+    bool                        HasScrollbarYCurr;          // Whether ANY instance of this table had a vertical scrollbar during the current frame.
+    bool                        HasScrollbarYPrev;          // Whether ANY instance of this table had a vertical scrollbar during the previous.
     bool                        MemoryCompacted;
     bool                        HostSkipItems;              // Backup of InnerWindow->SkipItem at the end of BeginTable(), because we will overwrite InnerWindow->SkipItem on a per-column basis
 

imgui_tables.cpp

@@ -395,6 +395,14 @@ bool    ImGui::BeginTableEx(const char* name, ImGuiID id, int columns_count, ImG
         table->OuterRect = table->InnerWindow->Rect();
         table->InnerRect = table->InnerWindow->InnerRect;
         IM_ASSERT(table->InnerWindow->WindowPadding.x == 0.0f && table->InnerWindow->WindowPadding.y == 0.0f && table->InnerWindow->WindowBorderSize == 0.0f);
+
+        // When using multiple instances, ensure they have the same amount of horizontal decorations (aka vertical scrollbar) so stretched columns can be aligned)
+        if (instance_no == 0)
+        {
+            table->HasScrollbarYPrev = table->HasScrollbarYCurr;
+            table->HasScrollbarYCurr = false;
+        }
+        table->HasScrollbarYCurr |= (table->InnerWindow->ScrollMax.y > 0.0f);
     }
     else
     {
@@ -893,7 +901,8 @@ void ImGui::TableUpdateLayout(ImGuiTable* table)
     // [Part 4] Apply final widths based on requested widths
     const ImRect work_rect = table->WorkRect;
     const float width_spacings = (table->OuterPaddingX * 2.0f) + (table->CellSpacingX1 + table->CellSpacingX2) * (table->ColumnsEnabledCount - 1);
-    const float width_avail = ((table->Flags & ImGuiTableFlags_ScrollX) && table->InnerWidth == 0.0f) ? table->InnerClipRect.GetWidth() : work_rect.GetWidth();
+    const float width_removed = (table->HasScrollbarYPrev && !table->InnerWindow->ScrollbarY) ? g.Style.ScrollbarSize : 0.0f; // To synchronize decoration width of synched tables with mismatching scrollbar state (#5920)
+    const float width_avail = ImMax(1.0f, (((table->Flags & ImGuiTableFlags_ScrollX) && table->InnerWidth == 0.0f) ? table->InnerClipRect.GetWidth() : work_rect.GetWidth()) - width_removed);
     const float width_avail_for_stretched_columns = width_avail - width_spacings - sum_width_requests;
     float width_remaining_for_stretched_columns = width_avail_for_stretched_columns;
     table->ColumnsGivenWidth = width_spacings + (table->CellPaddingX * 2.0f) * table->ColumnsEnabledCount;