Scrollbar: Fade out and disable interaction when too small, in order to facilitate using the resize grab on very small window, as well as reducing visual noise/overlap. (+1 squashed commits)

Internals: Added GetScrollbarID(). (#1185)

docs/CHANGELOG.txt

@@ -61,6 +61,8 @@ Other Changes:
 - RadioButton: Fixed label horizontal alignment to precisely match Checkbox().
 - Window: When resizing from an edge, the border is more visible and better follow the rounded corners.
 - Window: Fixed initial width of collapsed windows not taking account of contents width (broken in 1.67). (#2336, #176)
+- Scrollbar: Fade out and disable interaction when too small, in order to facilitate using the resize grab on very
+  small window, as well as reducing visual noise/overlap.
 - ListBox: Better optimized when clipped / non-visible.
 - InputTextMultiline: Better optimized when clipped / non-visible.
 - Font: Fixed high-level ImGui::CalcTextSize() used by most widgets from erroneously subtracting 1.0f*scale to

imgui_internal.h

@@ -1464,6 +1464,7 @@ namespace ImGui
     IMGUI_API bool          CollapseButton(ImGuiID id, const ImVec2& pos);
     IMGUI_API bool          ArrowButtonEx(const char* str_id, ImGuiDir dir, ImVec2 size_arg, ImGuiButtonFlags flags);
     IMGUI_API void          Scrollbar(ImGuiLayoutType direction);
+    IMGUI_API ImGuiID       GetScrollbarID(ImGuiLayoutType direction);
     IMGUI_API void          VerticalSeparator();        // Vertical separator, for menu bars (use current line height). Not exposed because it is misleading and it doesn't have an effect on regular layout.
 
     // Widgets low-level behaviors

imgui_widgets.cpp

@@ -710,6 +710,13 @@ bool ImGui::CollapseButton(ImGuiID id, const ImVec2& pos)
     return pressed;
 }
 
+ImGuiID ImGui::GetScrollbarID(ImGuiLayoutType direction)
+{
+    ImGuiContext& g = *GImGui;
+    ImGuiWindow* window = g.CurrentWindow;
+    return window->GetID((direction == ImGuiLayoutType_Horizontal) ? "#SCROLLX" : "#SCROLLY");
+}
+
 // Vertical/Horizontal scrollbar
 // The entire piece of code below is rather confusing because:
 // - We handle absolute seeking (when first clicking outside the grab) and relative manipulation (afterward or when clicking inside the grab)
@@ -722,7 +729,7 @@ void ImGui::Scrollbar(ImGuiLayoutType direction)
 
     const bool horizontal = (direction == ImGuiLayoutType_Horizontal);
     const ImGuiStyle& style = g.Style;
-    const ImGuiID id = window->GetID(horizontal ? "#SCROLLX" : "#SCROLLY");
+    const ImGuiID id = GetScrollbarID(direction);
     
     // Render background
     bool other_scrollbar = (horizontal ? window->ScrollbarY : window->ScrollbarX);
@@ -734,9 +741,21 @@ void ImGui::Scrollbar(ImGuiLayoutType direction)
         : ImRect(window_rect.Max.x - style.ScrollbarSize, window->Pos.y + border_size, window_rect.Max.x - border_size, window_rect.Max.y - other_scrollbar_size_w - border_size);
     if (!horizontal)
         bb.Min.y += window->TitleBarHeight() + ((window->Flags & ImGuiWindowFlags_MenuBar) ? window->MenuBarHeight() : 0.0f);
-    if (bb.GetWidth() <= 0.0f || bb.GetHeight() <= 0.0f)
+
+    const float bb_height = bb.GetHeight();
+    if (bb.GetWidth() <= 0.0f || bb_height <= 0.0f)
         return;
 
+    // When we are too small, start hiding and disabling the grab (this reduce visual noise on very small window and facilitate using the resize grab)
+    float alpha = 1.0f;
+    if ((direction == ImGuiLayoutType_Vertical) && bb_height < g.FontSize + g.Style.FramePadding.y * 2.0f)
+    {
+        alpha = ImSaturate((bb_height - g.FontSize) / (g.Style.FramePadding.y * 2.0f));
+        if (alpha <= 0.0f)
+            return;
+    }
+    const bool allow_interaction = (alpha >= 1.0f);
+
     int window_rounding_corners;
     if (horizontal)
         window_rounding_corners = ImDrawCornerFlags_BotLeft | (other_scrollbar ? 0 : ImDrawCornerFlags_BotRight);
@@ -767,7 +786,7 @@ void ImGui::Scrollbar(ImGuiLayoutType direction)
     float scroll_max = ImMax(1.0f, win_size_contents_v - win_size_avail_v);
     float scroll_ratio = ImSaturate(scroll_v / scroll_max);
     float grab_v_norm = scroll_ratio * (scrollbar_size_v - grab_h_pixels) / scrollbar_size_v;
-    if (held && grab_h_norm < 1.0f)
+    if (held && allow_interaction && grab_h_norm < 1.0f)
     {
         float scrollbar_pos_v = horizontal ? bb.Min.x : bb.Min.y;
         float mouse_pos_v = horizontal ? g.IO.MousePos.x : g.IO.MousePos.y;
@@ -810,8 +829,8 @@ void ImGui::Scrollbar(ImGuiLayoutType direction)
             *click_delta_to_grab_center_v = clicked_v_norm - grab_v_norm - grab_h_norm*0.5f;
     }
 
-    // Render
+    // Render grab
-    const ImU32 grab_col = GetColorU32(held ? ImGuiCol_ScrollbarGrabActive : hovered ? ImGuiCol_ScrollbarGrabHovered : ImGuiCol_ScrollbarGrab);
+    const ImU32 grab_col = GetColorU32(held ? ImGuiCol_ScrollbarGrabActive : hovered ? ImGuiCol_ScrollbarGrabHovered : ImGuiCol_ScrollbarGrab, alpha);
     ImRect grab_rect;
     if (horizontal)
         grab_rect = ImRect(ImLerp(bb.Min.x, bb.Max.x, grab_v_norm), bb.Min.y, ImMin(ImLerp(bb.Min.x, bb.Max.x, grab_v_norm) + grab_h_pixels, window_rect.Max.x), bb.Max.y);