Tab Bar: Fixed using more than 128 tabs in a tab bar.

Using ImS16 consistently + some better packing to avoid struct growing size.

docs/CHANGELOG.txt

@@ -55,6 +55,7 @@ Breaking Changes:
 Other Changes:
 
 - Tab Bar: Made it possible to append to an existing tab bar by calling BeginTabBar()/EndTabBar() again.
+- Tab Bar: Fixed using more than 128 tabs in a tab bar (scrolling policy recommended).
 - Drag and Drop: Fix drag and drop to tie same-size drop targets by choosen the later one. Fixes dragging
   into a full-window-sized dockspace inside a zero-padded window. (#3519, #2717) [@Black-Cat]
 - Backends: OpenGL: use glGetString(GL_VERSION) query instead of glGetIntegerv(GL_MAJOR_VERSION, ...) 

imgui_internal.h

@@ -1736,8 +1736,8 @@ struct ImGuiTabItem
     float               Width;                  // Width currently displayed
     float               ContentWidth;           // Width of label, stored during BeginTabItem() call
     ImS16               NameOffset;             // When Window==NULL, offset to name within parent ImGuiTabBar::TabsNames
-    ImS8                BeginOrder;             // BeginTabItem() order, used to re-order tabs after toggling ImGuiTabBarFlags_Reorderable
+    ImS16               BeginOrder;             // BeginTabItem() order, used to re-order tabs after toggling ImGuiTabBarFlags_Reorderable
-    ImS8                IndexDuringLayout;      // Index only used during TabBarLayout()
+    ImS16               IndexDuringLayout;      // Index only used during TabBarLayout()
     bool                WantClose;              // Marked as closed by SetTabItemClosed()
 
     ImGuiTabItem()      { ID = 0; Flags = ImGuiTabItemFlags_None; LastFrameVisible = LastFrameSelected = -1; NameOffset = -1; Offset = Width = ContentWidth = 0.0f; BeginOrder = -1; IndexDuringLayout = -1; WantClose = false; }
@@ -1767,14 +1767,14 @@ struct ImGuiTabBar
     float               ScrollingRectMaxX;
     ImGuiID             ReorderRequestTabId;
     ImS8                ReorderRequestDir;
-    ImS8                TabsActiveCount;        // Number of tabs submitted this frame.
-    ImS8                LastTabItemIdx;         // Index of last BeginTabItem() tab for use by EndTabItem()
     ImS8                BeginCount;
     bool                WantLayout;
     bool                VisibleTabWasSubmitted;
     bool                TabsAddedNew;           // Set to true when a new tab item or button has been added to the tab bar during last frame
+    ImS16               TabsActiveCount;        // Number of tabs submitted this frame.
+    ImS16               LastTabItemIdx;         // Index of last BeginTabItem() tab for use by EndTabItem()
+    float               ItemSpacingY;
     ImVec2              FramePadding;           // style.FramePadding locked at the time of BeginTabBar()
-    ImVec2              TabsContentsMin;
     ImVec2              BackupCursorPos;
     ImGuiTextBuffer     TabsNames;              // For non-docking tab bar we re-append names in a contiguous buffer.
 

imgui_widgets.cpp

@@ -6882,7 +6882,7 @@ bool    ImGui::BeginTabBarEx(ImGuiTabBar* tab_bar, const ImRect& tab_bar_bb, ImG
     tab_bar->BackupCursorPos = window->DC.CursorPos;
     if (tab_bar->CurrFrameVisible == g.FrameCount)
     {
-        window->DC.CursorPos = tab_bar->TabsContentsMin;
+        window->DC.CursorPos = ImVec2(tab_bar->BarRect.Min.x, tab_bar->BarRect.Max.y + tab_bar->ItemSpacingY);
         tab_bar->BeginCount++;
         return true;
     }
@@ -6904,14 +6904,13 @@ bool    ImGui::BeginTabBarEx(ImGuiTabBar* tab_bar, const ImRect& tab_bar_bb, ImG
     tab_bar->CurrFrameVisible = g.FrameCount;
     tab_bar->PrevTabsContentsHeight = tab_bar->CurrTabsContentsHeight;
     tab_bar->CurrTabsContentsHeight = 0.0f;
+    tab_bar->ItemSpacingY = g.Style.ItemSpacing.y;
     tab_bar->FramePadding = g.Style.FramePadding;
     tab_bar->TabsActiveCount = 0;
     tab_bar->BeginCount = 1;
 
     // Set cursor pos in a way which only be used in the off-chance the user erroneously submits item before BeginTabItem(): items will overlap
-    tab_bar->TabsContentsMin.x = tab_bar->BarRect.Min.x;
+    window->DC.CursorPos = ImVec2(tab_bar->BarRect.Min.x, tab_bar->BarRect.Max.y + tab_bar->ItemSpacingY);
-    tab_bar->TabsContentsMin.y = tab_bar->BarRect.Max.y + g.Style.ItemSpacing.y;
-    window->DC.CursorPos = tab_bar->TabsContentsMin;
 
     // Draw separator
     const ImU32 col = GetColorU32((flags & ImGuiTabBarFlags_IsFocused) ? ImGuiCol_TabActive : ImGuiCol_TabUnfocusedActive);
@@ -6990,7 +6989,7 @@ static void ImGui::TabBarLayout(ImGuiTabBar* tab_bar)
             tab_bar->Tabs[tab_dst_n] = tab_bar->Tabs[tab_src_n];
 
         tab = &tab_bar->Tabs[tab_dst_n];
-        tab->IndexDuringLayout = (ImS8)tab_dst_n;
+        tab->IndexDuringLayout = (ImS16)tab_dst_n;
 
         // We will need sorting if tabs have changed section (e.g. moved from one of Leading/Central/Trailing to another)
         int curr_tab_section_n = (tab->Flags & ImGuiTabItemFlags_Leading) ? 0 : (tab->Flags & ImGuiTabItemFlags_Trailing) ? 2 : 1;
@@ -7549,7 +7548,7 @@ bool    ImGui::TabItemEx(ImGuiTabBar* tab_bar, const char* label, bool* p_open,
         tab_bar->TabsAddedNew = true;
         tab_is_new = true;
     }
-    tab_bar->LastTabItemIdx = (ImS8)tab_bar->Tabs.index_from_ptr(tab);
+    tab_bar->LastTabItemIdx = (ImS16)tab_bar->Tabs.index_from_ptr(tab);
     tab->ContentWidth = size.x;
     tab->BeginOrder = tab_bar->TabsActiveCount++;