TabBar: feed desired width (sum of unclipped tabs width) into layout system to allow for auto-resize. (#2768)

Before 1.71 tab bars fed the sum of current width which created feedback loops in certain situations. Amend f95c77eeea.

docs/CHANGELOG.txt

@@ -39,6 +39,8 @@ Other Changes:
   when enabled will have small overlap glitches with (style.Alpha < 1.0).
 - TabBar: fixed ScrollToBar request creating bouncing loop when tab is larger than available space.
 - TabBar: fixed single-tab not shrinking their width down.
+- TabBar: feed desired width (sum of unclipped tabs width) into layout system to allow for auto-resize. (#2768)
+  (before 1.71 tab bars fed the sum of current width which created feedback loops in certain situations).
 - TabBar: improved shrinking for large number of tabs to avoid leaving extraneous space on the right side.
   Individuals tabs are given integer-rounded width and remainder is spread between tabs left-to-right.
 - SliderScalar: Improved assert when using U32 or U64 types with a large v_max value. (#2765) [@loicmouton]

imgui_demo.cpp

@@ -4286,7 +4286,6 @@ static void ShowExampleAppWindowTitles(bool*)
 // Demonstrate using the low-level ImDrawList to draw custom shapes.
 static void ShowExampleAppCustomRendering(bool* p_open)
 {
-    ImGui::SetNextWindowSize(ImVec2(350, 560), ImGuiCond_FirstUseEver);
     if (!ImGui::Begin("Example: Custom rendering", p_open))
     {
         ImGui::End();
@@ -4406,7 +4405,9 @@ static void ShowExampleAppCustomRendering(bool* p_open)
             static bool draw_bg = true;
             static bool draw_fg = true;
             ImGui::Checkbox("Draw in Background draw list", &draw_bg);
+            ImGui::SameLine(); HelpMarker("The Background draw list will be rendered below every Dear ImGui windows.");
             ImGui::Checkbox("Draw in Foreground draw list", &draw_fg);
+            ImGui::SameLine(); HelpMarker("The Foreground draw list will be rendered over every Dear ImGui windows.");
             ImVec2 window_pos = ImGui::GetWindowPos();
             ImVec2 window_size = ImGui::GetWindowSize();
             ImVec2 window_center = ImVec2(window_pos.x + window_size.x * 0.5f, window_pos.y + window_size.y * 0.5f);

imgui_internal.h

@@ -1435,8 +1435,9 @@ struct ImGuiTabBar
     int                 CurrFrameVisible;
     int                 PrevFrameVisible;
     ImRect              BarRect;
-    float               ContentsHeight;
+    float               LastTabContentHeight;   // Record the height of contents submitted below the tab bar
     float               OffsetMax;              // Distance from BarRect.Min.x, locked during layout
+    float               OffsetMaxIdeal;         // Ideal offset if all tabs were visible and not clipped
     float               OffsetNextTab;          // Distance from BarRect.Min.x, incremented with each BeginTabItem() call, not used if ImGuiTabBarFlags_Reorderable if set.
     float               ScrollingAnim;
     float               ScrollingTarget;

imgui_widgets.cpp

@@ -6290,8 +6290,8 @@ ImGuiTabBar::ImGuiTabBar()
     ID = 0;
     SelectedTabId = NextSelectedTabId = VisibleTabId = 0;
     CurrFrameVisible = PrevFrameVisible = -1;
-    ContentsHeight = 0.0f;
-    OffsetMax = OffsetNextTab = 0.0f;
+    LastTabContentHeight = 0.0f;
+    OffsetMax = OffsetMaxIdeal = OffsetNextTab = 0.0f;
     ScrollingAnim = ScrollingTarget = ScrollingTargetDistToVisibility = ScrollingSpeed = 0.0f;
     Flags = ImGuiTabBarFlags_None;
     ReorderRequestTabId = 0;
@@ -6373,7 +6373,7 @@ bool    ImGui::BeginTabBarEx(ImGuiTabBar* tab_bar, const ImRect& tab_bar_bb, ImG
     tab_bar->FramePadding = g.Style.FramePadding;
 
     // Layout
-    ItemSize(ImVec2(0.0f /*tab_bar->OffsetMax*/, tab_bar->BarRect.GetHeight())); // Don't feed width back
+    ItemSize(ImVec2(tab_bar->OffsetMaxIdeal, tab_bar->BarRect.GetHeight()));
     window->DC.CursorPos.x = tab_bar->BarRect.Min.x;
 
     // Draw separator
@@ -6406,9 +6406,9 @@ void    ImGui::EndTabBar()
     // Restore the last visible height if no tab is visible, this reduce vertical flicker/movement when a tabs gets removed without calling SetTabItemClosed().
     const bool tab_bar_appearing = (tab_bar->PrevFrameVisible + 1 < g.FrameCount);
     if (tab_bar->VisibleTabWasSubmitted || tab_bar->VisibleTabId == 0 || tab_bar_appearing)
-        tab_bar->ContentsHeight = ImMax(window->DC.CursorPos.y - tab_bar->BarRect.Max.y, 0.0f);
+        tab_bar->LastTabContentHeight = ImMax(window->DC.CursorPos.y - tab_bar->BarRect.Max.y, 0.0f);
     else
-        window->DC.CursorPos.y = tab_bar->BarRect.Max.y + tab_bar->ContentsHeight;
+        window->DC.CursorPos.y = tab_bar->BarRect.Max.y + tab_bar->LastTabContentHeight;
 
     if ((tab_bar->Flags & ImGuiTabBarFlags_DockNode) == 0)
         PopID();
@@ -6532,6 +6532,7 @@ static void ImGui::TabBarLayout(ImGuiTabBar* tab_bar)
 
     // Layout all active tabs
     float offset_x = initial_offset_x;
+    float offset_x_ideal = offset_x;
     tab_bar->OffsetNextTab = offset_x; // This is used by non-reorderable tab bar where the submission order is always honored.
     for (int tab_n = 0; tab_n < tab_bar->Tabs.Size; tab_n++)
     {
@@ -6540,8 +6541,10 @@ static void ImGui::TabBarLayout(ImGuiTabBar* tab_bar)
         if (scroll_track_selected_tab_id == 0 && g.NavJustMovedToId == tab->ID)
             scroll_track_selected_tab_id = tab->ID;
         offset_x += tab->Width + g.Style.ItemInnerSpacing.x;
+        offset_x_ideal += tab->WidthContents + g.Style.ItemInnerSpacing.x;
     }
     tab_bar->OffsetMax = ImMax(offset_x - g.Style.ItemInnerSpacing.x, 0.0f);
+    tab_bar->OffsetMaxIdeal = ImMax(offset_x_ideal - g.Style.ItemInnerSpacing.x, 0.0f);
 
     // Horizontal scrolling buttons
     const bool scrolling_buttons = (tab_bar->OffsetMax > tab_bar->BarRect.GetWidth() && tab_bar->Tabs.Size > 1) && !(tab_bar->Flags & ImGuiTabBarFlags_NoTabListScrollingButtons) && (tab_bar->Flags & ImGuiTabBarFlags_FittingPolicyScroll);