Tables: Fixed top-most and left-most outer border overlapping inner clip-rect when scrolling. (#6765)

docs/CHANGELOG.txt

@@ -107,6 +107,7 @@ Other changes:
     so a scrolling table can contribute to initial window size. (#6510)
   - Fixed subtle drawing overlap between borders in some situations.
   - Fixed bottom-most and right-most outer border offset by one. (#6765, #3752) [@v-ein]
+  - Fixed top-most and left-most outer border overlapping inner clip-rect when scrolling. (#6765)
   - Fixed top-most outer border being drawn with both TableBorderLight and TableBorderStrong
     in some situations, causing the earlier to be visible underneath when alpha is not 1.0f.
   - Fixed right-clicking right-most section (past right-most column) from highlighting a column.

imgui_tables.cpp

@@ -445,6 +445,18 @@ bool    ImGui::BeginTableEx(const char* name, ImGuiID id, int columns_count, ImG
     temp_data->HostBackupItemWidthStackSize = outer_window->DC.ItemWidthStack.Size;
     inner_window->DC.PrevLineSize = inner_window->DC.CurrLineSize = ImVec2(0.0f, 0.0f);
 
+    // Make left and top borders not overlap our contents by offsetting HostClipRect (#6765)
+    // (we normally shouldn't alter HostClipRect as we rely on TableMergeDrawChannels() expanding non-clipped column toward the
+    // limits of that rectangle, in order for ImDrawListSplitter::Merge() to merge the draw commands. However since the overlap
+    // problem only affect scrolling tables in this case we can get away with doing it without extra cost).
+    if (inner_window != outer_window)
+    {
+        if (flags & ImGuiTableFlags_BordersOuterV)
+            table->HostClipRect.Min.x = ImMin(table->HostClipRect.Min.x + TABLE_BORDER_SIZE, table->HostClipRect.Max.x);
+        if (flags & ImGuiTableFlags_BordersOuterH)
+            table->HostClipRect.Min.y = ImMin(table->HostClipRect.Min.y + TABLE_BORDER_SIZE, table->HostClipRect.Max.y);
+    }
+
     // Padding and Spacing
     // - None               ........Content..... Pad .....Content........
     // - PadOuter           | Pad ..Content..... Pad .....Content.. Pad |
@@ -1149,7 +1161,7 @@ void ImGui::TableUpdateLayout(ImGuiTable* table)
         table->InnerClipRect.Max.x = ImMin(table->InnerClipRect.Max.x, unused_x1);
     }
     table->InnerWindow->ParentWorkRect = table->WorkRect;
-    table->BorderX1 = table->InnerClipRect.Min.x + ((table->Flags & ImGuiTableFlags_BordersOuterV) ? 1.0f : 0.0f);
+    table->BorderX1 = table->InnerClipRect.Min.x;
     table->BorderX2 = table->InnerClipRect.Max.x;
 
     // Setup window's WorkRect.Max.y for GetContentRegionAvail(). Other values will be updated in each TableBeginCell() call.