TabBar: Tab-bars with ImGuiTabBarFlags_FittingPolicyScroll can be scrolled with horizontal mouse-wheel (or Shift + WheelY). (#2702)

docs/CHANGELOG.txt

@@ -52,6 +52,8 @@ Other changes:
 - Nav: Made Ctrl+Tab/Ctrl+Shift+Tab windowing register ownership to held modifier so
   it doesn't interfere with other code when remapping those actions. (#4828, #3255, #5641)
 - ColorEdit: Fixed shading of S/V triangle in Hue Wheel mode. (#5200, #6254) [@jamesthomasgriffin]
+- TabBar: Tab-bars with ImGuiTabBarFlags_FittingPolicyScroll can be scrolled with
+  horizontal mouse-wheel (or Shift + WheelY). (#2702)
 - Rendering: Using adaptative tesselation for: RadioButton, ColorEdit preview circles,
   Windows Close and Collapse Buttons.
 - Misc: Fixed ImVec2 operator[] violating aliasing rules causing issue with Intel C++

imgui.cpp

@@ -3867,7 +3867,7 @@ void ImGui::MarkItemEdited(ImGuiID id)
     g.LastItemData.StatusFlags |= ImGuiItemStatusFlags_Edited;
 }
 
-static inline bool IsWindowContentHoverable(ImGuiWindow* window, ImGuiHoveredFlags flags)
+bool ImGui::IsWindowContentHoverable(ImGuiWindow* window, ImGuiHoveredFlags flags)
 {
     // An active popup disable hovering on other windows (apart from its own children)
     // FIXME-OPT: This could be cached/stored within the window.

imgui_internal.h

@@ -2828,6 +2828,7 @@ namespace ImGui
     inline void             ItemSize(const ImRect& bb, float text_baseline_y = -1.0f) { ItemSize(bb.GetSize(), text_baseline_y); } // FIXME: This is a misleading API since we expect CursorPos to be bb.Min.
     IMGUI_API bool          ItemAdd(const ImRect& bb, ImGuiID id, const ImRect* nav_bb = NULL, ImGuiItemFlags extra_flags = 0);
     IMGUI_API bool          ItemHoverable(const ImRect& bb, ImGuiID id);
+    IMGUI_API bool          IsWindowContentHoverable(ImGuiWindow* window, ImGuiHoveredFlags flags = 0);
     IMGUI_API bool          IsClippedEx(const ImRect& bb, ImGuiID id);
     IMGUI_API void          SetLastItemData(ImGuiID item_id, ImGuiItemFlags in_flags, ImGuiItemStatusFlags status_flags, const ImRect& item_rect);
     IMGUI_API ImVec2        CalcItemSize(ImVec2 size, float default_w, float default_h);

imgui_widgets.cpp

@@ -7618,6 +7618,12 @@ void    ImGui::EndTabBar()
     g.CurrentTabBar = g.CurrentTabBarStack.empty() ? NULL : GetTabBarFromTabBarRef(g.CurrentTabBarStack.back());
 }
 
+// Scrolling happens only in the central section (leading/trailing sections are not scrolling)
+static float TabBarCalcScrollableWidth(ImGuiTabBar* tab_bar, ImGuiTabBarSection* sections)
+{
+    return tab_bar->BarRect.GetWidth() - sections[0].Width - sections[2].Width - sections[1].Spacing;
+}
+
 // This is called only once a frame before by the first call to ItemTab()
 // The reason we're not calling it in BeginTabBar() is to leave a chance to the user to call the SetTabItemClosed() functions.
 static void ImGui::TabBarLayout(ImGuiTabBar* tab_bar)
@@ -7820,9 +7826,23 @@ static void ImGui::TabBarLayout(ImGuiTabBar* tab_bar)
     tab_bar->VisibleTabId = tab_bar->SelectedTabId;
     tab_bar->VisibleTabWasSubmitted = false;
 
-    // Update scrolling
+    // Apply request requests
     if (scroll_to_tab_id != 0)
         TabBarScrollToTab(tab_bar, scroll_to_tab_id, sections);
+    else if ((tab_bar->Flags & ImGuiTabBarFlags_FittingPolicyScroll) && IsMouseHoveringRect(tab_bar->BarRect.Min, tab_bar->BarRect.Max, true) && IsWindowContentHoverable(g.CurrentWindow))
+    {
+        const float wheel = g.IO.MouseWheelRequestAxisSwap ? g.IO.MouseWheel : g.IO.MouseWheelH;
+        const ImGuiKey wheel_key = g.IO.MouseWheelRequestAxisSwap ? ImGuiKey_MouseWheelY : ImGuiKey_MouseWheelX;
+        if (TestKeyOwner(wheel_key, tab_bar->ID) && wheel != 0.0f)
+        {
+            const float scroll_step = wheel * TabBarCalcScrollableWidth(tab_bar, sections) / 3.0f;
+            tab_bar->ScrollingTargetDistToVisibility = 0.0f;
+            tab_bar->ScrollingTarget = TabBarScrollClamp(tab_bar, tab_bar->ScrollingTarget - scroll_step);
+        }
+        SetKeyOwner(wheel_key, tab_bar->ID);
+    }
+
+    // Update scrolling
     tab_bar->ScrollingAnim = TabBarScrollClamp(tab_bar, tab_bar->ScrollingAnim);
     tab_bar->ScrollingTarget = TabBarScrollClamp(tab_bar, tab_bar->ScrollingTarget);
     if (tab_bar->ScrollingAnim != tab_bar->ScrollingTarget)
@@ -7959,8 +7979,7 @@ static void ImGui::TabBarScrollToTab(ImGuiTabBar* tab_bar, ImGuiID tab_id, ImGui
     int order = TabBarGetTabOrder(tab_bar, tab);
 
     // Scrolling happens only in the central section (leading/trailing sections are not scrolling)
-    // FIXME: This is all confusing.
+    float scrollable_width = TabBarCalcScrollableWidth(tab_bar, sections);
-    float scrollable_width = tab_bar->BarRect.GetWidth() - sections[0].Width - sections[2].Width - sections[1].Spacing;
 
     // We make all tabs positions all relative Sections[0].Width to make code simpler
     float tab_x1 = tab->Offset - sections[0].Width + (order > sections[0].TabCount - 1 ? -margin : 0.0f);