Columns: Lower overhead on column switches and switching to background channel (some stress tests in debug builds went 3->2 ms). (#125)

This change benefits Columns but was primarily made with Tables in mind.

docs/CHANGELOG.txt

@@ -46,6 +46,8 @@ Other Changes:
   flag was also set, and _OpenOnArrow is frequently set along with _OpenOnDoubleClick).
 - TreeNode: Fixed bug where dragging a payload over a TreeNode() with either _OpenOnDoubleClick
   or _OpenOnArrow would open the node. (#143)
+- Columns: Lower overhead on column switches and switching to background channel (some of our stress
+  tests in debug builds went 3->2 ms). Benefits Columns but was primarily made with Tables in mind!
 - Style: Added style.TabMinWidthForUnselectedCloseButton settings.
   Set to 0.0f (default) to always make a close button appear on hover (same as Chrome, VS).
   Set to FLT_MAX to only display a close button when selected (merely hovering is not enough).

imgui.cpp

@@ -4167,7 +4167,12 @@ static void SetupDrawData(ImVector<ImDrawList*>* draw_lists, ImDrawData* draw_da
     }
 }
 
-// When using this function it is sane to ensure that float are perfectly rounded to integer values, to that e.g. (int)(max.x-min.x) in user's render produce correct result.
+// Push a clipping rectangle for both ImGui logic (hit-testing etc.) and low-level ImDrawList rendering.
+// - When using this function it is sane to ensure that float are perfectly rounded to integer values,
+//   so that e.g. (int)(max.x-min.x) in user's render produce correct result.
+// - If the code here changes, may need to update code of functions like NextColumn() and PushColumnClipRect():
+//   some frequently called functions which to modify both channels and clipping simultaneously tend to use a more
+//   specialized code path to added extraneous updates of the underlying ImDrawCmd.
 void ImGui::PushClipRect(const ImVec2& clip_rect_min, const ImVec2& clip_rect_max, bool intersect_with_current_clip_rect)
 {
     ImGuiWindow* window = GetCurrentWindow();

imgui_internal.h

@@ -435,6 +435,7 @@ struct IMGUI_API ImRect
     void        ClipWithFull(const ImRect& r)       { Min = ImClamp(Min, r.Min, r.Max); Max = ImClamp(Max, r.Min, r.Max); } // Full version, ensure both points are fully clipped.
     void        Floor()                             { Min.x = IM_FLOOR(Min.x); Min.y = IM_FLOOR(Min.y); Max.x = IM_FLOOR(Max.x); Max.y = IM_FLOOR(Max.y); }
     bool        IsInverted() const                  { return Min.x > Max.x || Min.y > Max.y; }
+    ImVec4      ToVec4() const                      { return ImVec4(Min.x, Min.y, Max.x, Max.y); }
 };
 
 // Helper: ImBitArray

imgui_widgets.cpp

@@ -7549,6 +7549,11 @@ void ImGui::PushColumnsBackground()
     ImGuiColumns* columns = window->DC.CurrentColumns;
     if (columns->Count == 1)
         return;
+
+    // Set cmd header ahead to avoid SetCurrentChannel+PushClipRect doing an unnecessary AddDrawCmd/Pop
+    //if (window->DrawList->Flags & ImDrawListFlags_Debug) IMGUI_DEBUG_LOG("PushColumnsBackground()\n");
+    window->DrawList->_CmdHeader.ClipRect = columns->HostClipRect.ToVec4();
+
     columns->Splitter.SetCurrentChannel(window->DrawList, 0);
     int cmd_size = window->DrawList->CmdBuffer.Size;
     PushClipRect(columns->HostClipRect.Min, columns->HostClipRect.Max, false);
@@ -7562,6 +7567,12 @@ void ImGui::PopColumnsBackground()
     ImGuiColumns* columns = window->DC.CurrentColumns;
     if (columns->Count == 1)
         return;
+
+    // Set cmd header ahead to avoid SetCurrentChannel+PushClipRect doing an unnecessary AddDrawCmd/Pop
+    //if (window->DrawList->Flags & ImDrawListFlags_Debug) IMGUI_DEBUG_LOG("PopColumnsBackground()\n");
+    ImVec4 pop_clip_rect = window->DrawList->_ClipRectStack.Data[window->DrawList->_ClipRectStack.Size - 2];
+    window->DrawList->_CmdHeader.ClipRect = pop_clip_rect;
+
     columns->Splitter.SetCurrentChannel(window->DrawList, columns->Current + 1);
     PopClipRect();
 }
@@ -7684,11 +7695,22 @@ void ImGui::NextColumn()
         return;
     }
     PopItemWidth();
-    PopClipRect();
+
+    // Next column
+    if (++columns->Current == columns->Count)
+        columns->Current = 0;
+
+    // As a small optimization, to avoid doing PopClipRect() + SetCurrentChannel() + PushClipRect()
+    // (which would needlessly attempt to update commands in the wrong channel, then pop or overwrite them),
+    // We use a shortcut: we override ClipRect in window and drawlist's CmdHeader + SetCurrentChannel().
+    ImGuiColumnData* column = &columns->Columns[columns->Current];
+    window->ClipRect = column->ClipRect;
+    window->DrawList->_CmdHeader.ClipRect = column->ClipRect.ToVec4();
+    //PopClipRect();
 
     const float column_padding = g.Style.ItemSpacing.x;
     columns->LineMaxY = ImMax(columns->LineMaxY, window->DC.CursorPos.y);
-    if (++columns->Current < columns->Count)
+    if (columns->Current > 0)
     {
         // Columns 1+ ignore IndentX (by canceling it out)
         // FIXME-COLUMNS: Unnecessary, could be locked?
@@ -7701,7 +7723,6 @@ void ImGui::NextColumn()
         // Column 0 honor IndentX
         window->DC.ColumnsOffset.x = ImMax(column_padding - window->WindowPadding.x, 0.0f);
         columns->Splitter.SetCurrentChannel(window->DrawList, 1);
-        columns->Current = 0;
         columns->LineMinY = columns->LineMaxY;
     }
     window->DC.CursorPos.x = IM_FLOOR(window->Pos.x + window->DC.Indent.x + window->DC.ColumnsOffset.x);
@@ -7709,7 +7730,7 @@ void ImGui::NextColumn()
     window->DC.CurrLineSize = ImVec2(0.0f, 0.0f);
     window->DC.CurrLineTextBaseOffset = 0.0f;
 
-    PushColumnClipRect(columns->Current);     // FIXME-COLUMNS: Could it be an overwrite?
+    //PushColumnClipRect(columns->Current);
 
     // FIXME-COLUMNS: Share code with BeginColumns() - move code on columns setup.
     float offset_0 = GetColumnOffset(columns->Current);