Scrollbar: stabilize visibility of ScrollbarX when detecting a feedback loop. (#8488, #3285, #4539)

docs/CHANGELOG.txt

@@ -82,6 +82,14 @@ Other changes:
   visibility of the preview/hint buffer. (#8368) [@m9710797, @ocornut]
 - Scrollbar: Rework logic that fades-out scrollbar when it becomes too small,
   which amusingly made it disappear when using very big font/frame size.
+- Scrollbar: Automatically stabilize ScrollbarX visibility when detecting a
+  feedback loop manifesting with ScrollbarX visibility toggling on and off
+  repeatedly. (#8488, #3285, #4539)
+  (feedback loops of this sort can manifest in various situations, but combining
+  horizontal + vertical scrollbar + using a clipper with varying width items is
+  one frequent cause. The better solution is to, either: (1) enforce visibility
+  by using ImGuiWindowFlags_AlwaysHorizontalScrollbar or (2) declare stable
+  contents width with SetNextWindowContentSize(), if you can compute it.)
 - Tables: fixed calling SetNextWindowScroll() on clipped scrolling table
   to not leak the value into a subsequent window. (#8196)
 - Tables: fixed an issue where Columns Visible/Width state wouldn't be correctly

imgui.cpp

@@ -7456,9 +7456,23 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
             ImVec2 needed_size_from_last_frame = window_just_created ? ImVec2(0, 0) : window->ContentSize + window->WindowPadding * 2.0f;
             float size_x_for_scrollbars = use_current_size_for_scrollbar_x ? avail_size_from_current_frame.x : avail_size_from_last_frame.x;
             float size_y_for_scrollbars = use_current_size_for_scrollbar_y ? avail_size_from_current_frame.y : avail_size_from_last_frame.y;
+            bool scrollbar_x_prev = window->ScrollbarX;
             //bool scrollbar_y_from_last_frame = window->ScrollbarY; // FIXME: May want to use that in the ScrollbarX expression? How many pros vs cons?
             window->ScrollbarY = (flags & ImGuiWindowFlags_AlwaysVerticalScrollbar) || ((needed_size_from_last_frame.y > size_y_for_scrollbars) && !(flags & ImGuiWindowFlags_NoScrollbar));
             window->ScrollbarX = (flags & ImGuiWindowFlags_AlwaysHorizontalScrollbar) || ((needed_size_from_last_frame.x > size_x_for_scrollbars - (window->ScrollbarY ? style.ScrollbarSize : 0.0f)) && !(flags & ImGuiWindowFlags_NoScrollbar) && (flags & ImGuiWindowFlags_HorizontalScrollbar));
+
+            // Track when ScrollbarX visibility keeps toggling, which is a sign of a feedback loop, and stabilize by enforcing visibility (#3285, #8488)
+            // (Feedback loops of this sort can manifest in various situations, but combining horizontal + vertical scrollbar + using a clipper with varying width items is one frequent cause.
+            //  The better solution is to, either (1) enforce visibility by using ImGuiWindowFlags_AlwaysHorizontalScrollbar or (2) declare stable contents width with SetNextWindowContentSize(), if you can compute it)
+            window->ScrollbarXStabilizeToggledHistory <<= 1;
+            window->ScrollbarXStabilizeToggledHistory |= (scrollbar_x_prev != window->ScrollbarX) ? 0x01 : 0x00;
+            const bool scrollbar_x_stabilize = (window->ScrollbarXStabilizeToggledHistory != 0) && ImCountSetBits(window->ScrollbarXStabilizeToggledHistory) >= 4; // 4 == half of bits in our U8 history.
+            if (scrollbar_x_stabilize)
+                window->ScrollbarX = true;
+            //if (scrollbar_x_stabilize && !window->ScrollbarXStabilizeEnabled)
+            //    IMGUI_DEBUG_LOG("[scroll] Stabilize ScrollbarX for Window '%s'\n", window->Name);
+            window->ScrollbarXStabilizeEnabled = scrollbar_x_stabilize;
+
             if (window->ScrollbarX && !window->ScrollbarY)
                 window->ScrollbarY = (needed_size_from_last_frame.y > size_y_for_scrollbars - style.ScrollbarSize) && !(flags & ImGuiWindowFlags_NoScrollbar);
             window->ScrollbarSizes = ImVec2(window->ScrollbarY ? style.ScrollbarSize : 0.0f, window->ScrollbarX ? style.ScrollbarSize : 0.0f);

imgui_internal.h

@@ -367,9 +367,10 @@ static inline void      ImQsort(void* base, size_t count, size_t size_of_element
 IMGUI_API ImU32         ImAlphaBlendColors(ImU32 col_a, ImU32 col_b);
 
 // Helpers: Bit manipulation
-static inline bool      ImIsPowerOfTwo(int v)           { return v != 0 && (v & (v - 1)) == 0; }
-static inline bool      ImIsPowerOfTwo(ImU64 v)         { return v != 0 && (v & (v - 1)) == 0; }
-static inline int       ImUpperPowerOfTwo(int v)        { v--; v |= v >> 1; v |= v >> 2; v |= v >> 4; v |= v >> 8; v |= v >> 16; v++; return v; }
+static inline bool      ImIsPowerOfTwo(int v)               { return v != 0 && (v & (v - 1)) == 0; }
+static inline bool      ImIsPowerOfTwo(ImU64 v)             { return v != 0 && (v & (v - 1)) == 0; }
+static inline int       ImUpperPowerOfTwo(int v)            { v--; v |= v >> 1; v |= v >> 2; v |= v >> 4; v |= v >> 8; v |= v >> 16; v++; return v; }
+static inline unsigned int ImCountSetBits(unsigned int v)   { unsigned int count = 0; while (v > 0) { v = v & (v - 1); count++; } return count; }
 
 // Helpers: String
 #define ImStrlen strlen
@@ -2514,6 +2515,8 @@ struct IMGUI_API ImGuiWindow
     ImVec2                  ScrollTargetEdgeSnapDist;           // 0.0f = no snapping, >0.0f snapping threshold
     ImVec2                  ScrollbarSizes;                     // Size taken by each scrollbars on their smaller axis. Pay attention! ScrollbarSizes.x == width of the vertical scrollbar, ScrollbarSizes.y = height of the horizontal scrollbar.
     bool                    ScrollbarX, ScrollbarY;             // Are scrollbars visible?
+    bool                    ScrollbarXStabilizeEnabled;         // Was ScrollbarX previously auto-stabilized?
+    ImU8                    ScrollbarXStabilizeToggledHistory;  // Used to stabilize scrollbar visibility in case of feedback loops
     bool                    Active;                             // Set to true on Begin(), unless Collapsed
     bool                    WasActive;
     bool                    WriteAccessed;                      // Set to true when any widget access the current window