TabBar: made a change to that declared ideal width (for auto-resize) won't lag by an extra frame.

Vaguely relate to underlying (uncommited) work for #3291

imgui_widgets.cpp

diff --git a/imgui_widgets.cpp b/imgui_widgets.cpp
index 1d4362d..1378613 100644
--- a/imgui_widgets.cpp
+++ b/imgui_widgets.cpp
@@ -6657,9 +6657,9 @@
     tab_bar->CurrFrameVisible = g.FrameCount;
     tab_bar->FramePadding = g.Style.FramePadding;
 
-    // Layout
-    ItemSize(ImVec2(tab_bar->OffsetMaxIdeal, tab_bar->BarRect.GetHeight()), tab_bar->FramePadding.y);
+    // Set cursor pos in a way which only be used in the off-chance the user erroneously submits item before BeginTabItem(): items will overlap
     window->DC.CursorPos.x = tab_bar->BarRect.Min.x;
+    window->DC.CursorPos.y = tab_bar->BarRect.Max.y + g.Style.ItemSpacing.y;
 
     // Draw separator
     const ImU32 col = GetColorU32((flags & ImGuiTabBarFlags_IsFocused) ? ImGuiCol_TabActive : ImGuiCol_TabUnfocusedActive);
@@ -6685,7 +6685,7 @@
         IM_ASSERT_USER_ERROR(tab_bar != NULL, "Mismatched BeginTabBar()/EndTabBar()!");
         return;
     }
-    if (tab_bar->WantLayout)
+    if (tab_bar->WantLayout) // Fallback in case no TabItem have been submitted
         TabBarLayout(tab_bar);
 
     // Restore the last visible height if no tab is visible, this reduce vertical flicker/movement when a tabs gets removed without calling SetTabItemClosed().
@@ -6871,6 +6871,11 @@
     // Clear name buffers
     if ((tab_bar->Flags & ImGuiTabBarFlags_DockNode) == 0)
         tab_bar->TabsNames.Buf.resize(0);
+
+    // Actual layout in host window (we don't do it in BeginTabBar() so as not to waste an extra frame)
+    ImGuiWindow* window = g.CurrentWindow;
+    window->DC.CursorPos = tab_bar->BarRect.Min;
+    ItemSize(ImVec2(tab_bar->OffsetMaxIdeal, tab_bar->BarRect.GetHeight()), tab_bar->FramePadding.y);
 }
 
 // Dockables uses Name/ID in the global namespace. Non-dockable items use the ID stack.