Scrolling: Avoid SetScroll, SetScrollFromPos functions from snapping on the edge of scroll limits. (#3379) + Demo: Rename "Layout" to "Layout & Scrolling".

Author: ocornut

docs/CHANGELOG.txt

@@ -49,6 +49,10 @@ Other Changes:
   clipping, more than 16 KB characters are visible in the same low-level ImDrawList::RenderText
   call. ImGui-level functions such as TextUnformatted() are not affected. This is quite rare
   but it will be addressed later). (#3349)
+- Scrolling: Avoid SetScroll, SetScrollFromPos functions from snapping on the edge of scroll
+  limits when close-enough by (WindowPadding - ItemPadding), which was a tweak with too many
+  side-effects. The behavior is still present in SetScrollHere functions as they are more explicitly
+  aiming at making widgets visible. May later be moved to a flag.
 - InvisibleButton: Made public a small selection of ImGuiButtonFlags (previously in imgui_internal.h)
   and allowed to pass them to InvisibleButton(): ImGuiButtonFlags_MouseButtonLeft/Right/Middle.
   This is a small but rather important change because lots of multi-button behaviors could previously

imgui.cpp

@@ -805,7 +805,7 @@ static const float WINDOWS_MOUSE_WHEEL_SCROLL_LOCK_TIMER    = 2.00f;    // Lock
 static void             SetCurrentWindow(ImGuiWindow* window);
 static void             FindHoveredWindow();
 static ImGuiWindow*     CreateNewWindow(const char* name, ImGuiWindowFlags flags);
-static ImVec2           CalcNextScrollFromScrollTargetAndClamp(ImGuiWindow* window, bool snap_on_edges);
+static ImVec2           CalcNextScrollFromScrollTargetAndClamp(ImGuiWindow* window);
 
 static void             AddDrawListToDrawData(ImVector<ImDrawList*>* out_list, ImDrawList* draw_list);
 static void             AddWindowToSortBuffer(ImVector<ImGuiWindow*>* out_sorted_windows, ImGuiWindow* window);
@@ -5819,7 +5819,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         window->ScrollMax.y = ImMax(0.0f, window->ContentSize.y + window->WindowPadding.y * 2.0f - window->InnerRect.GetHeight());
 
         // Apply scrolling
-        window->Scroll = CalcNextScrollFromScrollTargetAndClamp(window, true);
+        window->Scroll = CalcNextScrollFromScrollTargetAndClamp(window);
         window->ScrollTarget = ImVec2(FLT_MAX, FLT_MAX);
 
         // DRAWING
@@ -7325,30 +7325,20 @@ void ImGui::EndGroup()
 // [SECTION] SCROLLING
 //-----------------------------------------------------------------------------
 
-static ImVec2 CalcNextScrollFromScrollTargetAndClamp(ImGuiWindow* window, bool snap_on_edges)
+static ImVec2 CalcNextScrollFromScrollTargetAndClamp(ImGuiWindow* window)
 {
-    ImGuiContext& g = *GImGui;
     ImVec2 scroll = window->Scroll;
     if (window->ScrollTarget.x < FLT_MAX)
     {
         float cr_x = window->ScrollTargetCenterRatio.x;
         float target_x = window->ScrollTarget.x;
-        if (snap_on_edges && cr_x <= 0.0f && target_x <= window->WindowPadding.x)
-            target_x = 0.0f;
-        else if (snap_on_edges && cr_x >= 1.0f && target_x >= window->ContentSize.x + window->WindowPadding.x + g.Style.ItemSpacing.x)
-            target_x = window->ContentSize.x + window->WindowPadding.x * 2.0f;
         scroll.x = target_x - cr_x * (window->SizeFull.x - window->ScrollbarSizes.x);
     }
     if (window->ScrollTarget.y < FLT_MAX)
     {
-        // 'snap_on_edges' allows for a discontinuity at the edge of scrolling limits to take account of WindowPadding so that scrolling to make the last item visible scroll far enough to see the padding.
         float decoration_up_height = window->TitleBarHeight() + window->MenuBarHeight();
         float cr_y = window->ScrollTargetCenterRatio.y;
         float target_y = window->ScrollTarget.y;
-        if (snap_on_edges && cr_y <= 0.0f && target_y <= window->WindowPadding.y)
-            target_y = 0.0f;
-        if (snap_on_edges && cr_y >= 1.0f && target_y >= window->ContentSize.y + window->WindowPadding.y + g.Style.ItemSpacing.y)
-            target_y = window->ContentSize.y + window->WindowPadding.y * 2.0f;
         scroll.y = target_y - cr_y * (window->SizeFull.y - window->ScrollbarSizes.y - decoration_up_height);
     }
     scroll.x = IM_FLOOR(ImMax(scroll.x, 0.0f));
@@ -7380,7 +7370,7 @@ ImVec2 ImGui::ScrollToBringRectIntoView(ImGuiWindow* window, const ImRect& item_
         else if (item_rect.Max.y >= window_rect.Max.y)
             SetScrollFromPosY(window, item_rect.Max.y - window->Pos.y + g.Style.ItemSpacing.y, 1.0f);
 
-        ImVec2 next_scroll = CalcNextScrollFromScrollTargetAndClamp(window, false);
+        ImVec2 next_scroll = CalcNextScrollFromScrollTargetAndClamp(window);
         delta_scroll = next_scroll - window->Scroll;
     }
 
@@ -7441,21 +7431,19 @@ void ImGui::SetScrollY(ImGuiWindow* window, float new_scroll_y)
     window->ScrollTargetCenterRatio.y = 0.0f;
 }
 
-
+// Note that a local position will vary depending on initial scroll value
+// We store a target position so centering can occur on the next frame when we are guaranteed to have a known window size
 void ImGui::SetScrollFromPosX(ImGuiWindow* window, float local_x, float center_x_ratio)
 {
-    // We store a target position so centering can occur on the next frame when we are guaranteed to have a known window size
     IM_ASSERT(center_x_ratio >= 0.0f && center_x_ratio <= 1.0f);
     window->ScrollTarget.x = IM_FLOOR(local_x + window->Scroll.x);
     window->ScrollTargetCenterRatio.x = center_x_ratio;
 }
 
 void ImGui::SetScrollFromPosY(ImGuiWindow* window, float local_y, float center_y_ratio)
 {
-    // We store a target position so centering can occur on the next frame when we are guaranteed to have a known window size
     IM_ASSERT(center_y_ratio >= 0.0f && center_y_ratio <= 1.0f);
-    const float decoration_up_height = window->TitleBarHeight() + window->MenuBarHeight();
-    local_y -= decoration_up_height;
+    local_y -= window->TitleBarHeight() + window->MenuBarHeight(); // FIXME: Would be nice to have a more standardized access to our scrollable/client rect
     window->ScrollTarget.y = IM_FLOOR(local_y + window->Scroll.y);
     window->ScrollTargetCenterRatio.y = center_y_ratio;
 }
@@ -7472,25 +7460,51 @@ void ImGui::SetScrollFromPosY(float local_y, float center_y_ratio)
     SetScrollFromPosY(g.CurrentWindow, local_y, center_y_ratio);
 }
 
+// Tweak: snap on edges when aiming at an item very close to the edge,
+// So the difference between WindowPadding and ItemSpacing will be in the visible area after scrolling.
+// When we refactor the scrolling API this may be configurable with a flag?
+// Note that the effect for this won't be visible on X axis with default Style settings as WindowPadding.x == ItemSpacing.x by default.
+static float CalcScrollSnap(float target, float snap_min, float snap_max, float snap_threshold, float center_ratio)
+{
+    if (target <= snap_min + snap_threshold)
+        return ImLerp(snap_min, target, center_ratio);
+    if (target >= snap_max - snap_threshold)
+        return ImLerp(target, snap_max, center_ratio);
+    return target;
+}
+
 // center_x_ratio: 0.0f left of last item, 0.5f horizontal center of last item, 1.0f right of last item.
 void ImGui::SetScrollHereX(float center_x_ratio)
 {
     ImGuiContext& g = *GImGui;
     ImGuiWindow* window = g.CurrentWindow;
-    float target_x = window->DC.LastItemRect.Min.x - window->Pos.x; // Left of last item, in window space
-    float last_item_width = window->DC.LastItemRect.GetWidth();
-    target_x += (last_item_width * center_x_ratio) + (g.Style.ItemSpacing.x * (center_x_ratio - 0.5f) * 2.0f); // Precisely aim before, in the middle or after the last item.
-    SetScrollFromPosX(target_x, center_x_ratio);
+    float spacing_x = g.Style.ItemSpacing.x;
+    float target_x = ImLerp(window->DC.LastItemRect.Min.x - spacing_x, window->DC.LastItemRect.Max.x + spacing_x, center_x_ratio);
+
+    // Tweak: snap on edges when aiming at an item very close to the edge
+    const float snap_x_threshold = ImMax(0.0f, window->WindowPadding.x - spacing_x);
+    const float snap_x_min = window->DC.CursorStartPos.x - window->WindowPadding.x;
+    const float snap_x_max = window->DC.CursorStartPos.x + window->ContentSize.x + window->WindowPadding.x;
+    target_x = CalcScrollSnap(target_x, snap_x_min, snap_x_max, snap_x_threshold, center_x_ratio);
+
+    SetScrollFromPosX(window, target_x - window->Pos.x, center_x_ratio);
 }
 
 // center_y_ratio: 0.0f top of last item, 0.5f vertical center of last item, 1.0f bottom of last item.
 void ImGui::SetScrollHereY(float center_y_ratio)
 {
     ImGuiContext& g = *GImGui;
     ImGuiWindow* window = g.CurrentWindow;
-    float target_y = window->DC.CursorPosPrevLine.y - window->Pos.y; // Top of last item, in window space
-    target_y += (window->DC.PrevLineSize.y * center_y_ratio) + (g.Style.ItemSpacing.y * (center_y_ratio - 0.5f) * 2.0f); // Precisely aim above, in the middle or below the last line.
-    SetScrollFromPosY(target_y, center_y_ratio);
+    float spacing_y = g.Style.ItemSpacing.y;
+    float target_y = ImLerp(window->DC.CursorPosPrevLine.y - spacing_y, window->DC.CursorPosPrevLine.y + window->DC.PrevLineSize.y + spacing_y, center_y_ratio);
+
+    // Tweak: snap on edges when aiming at an item very close to the edge
+    const float snap_y_threshold = ImMax(0.0f, window->WindowPadding.y - spacing_y);
+    const float snap_y_min = window->DC.CursorStartPos.y - window->WindowPadding.y;
+    const float snap_y_max = window->DC.CursorStartPos.y + window->ContentSize.y + window->WindowPadding.y;
+    target_y = CalcScrollSnap(target_y, snap_y_min, snap_y_max, snap_y_threshold, center_y_ratio);
+
+    SetScrollFromPosY(window, target_y - window->Pos.y, center_y_ratio);
 }
 
 //-----------------------------------------------------------------------------

imgui.h

@@ -333,8 +333,8 @@ namespace ImGui
     // Windows Scrolling
     IMGUI_API float         GetScrollX();                                                   // get scrolling amount [0..GetScrollMaxX()]
     IMGUI_API float         GetScrollY();                                                   // get scrolling amount [0..GetScrollMaxY()]
-    IMGUI_API float         GetScrollMaxX();                                                // get maximum scrolling amount ~~ ContentSize.X - WindowSize.X
-    IMGUI_API float         GetScrollMaxY();                                                // get maximum scrolling amount ~~ ContentSize.Y - WindowSize.Y
+    IMGUI_API float         GetScrollMaxX();                                                // get maximum scrolling amount ~~ ContentSize.x - WindowSize.x
+    IMGUI_API float         GetScrollMaxY();                                                // get maximum scrolling amount ~~ ContentSize.y - WindowSize.y
     IMGUI_API void          SetScrollX(float scroll_x);                                     // set scrolling amount [0..GetScrollMaxX()]
     IMGUI_API void          SetScrollY(float scroll_y);                                     // set scrolling amount [0..GetScrollMaxY()]
     IMGUI_API void          SetScrollHereX(float center_x_ratio = 0.5f);                    // adjust scrolling amount to make current cursor position visible. center_x_ratio=0.0: left, 0.5: center, 1.0: right. When using to make a "default/current item" visible, consider using SetItemDefaultFocus() instead.

imgui_demo.cpp

@@ -2490,11 +2490,9 @@ static void ShowDemoWindowLayout()
         ImGui::Spacing();
         HelpMarker(
             "Use SetScrollHereX() or SetScrollFromPosX() to scroll to a given horizontal position.\n\n"
-            "Using the \"Scroll To Pos\" button above will make the discontinuity at edges visible: "
-            "scrolling to the top/bottom/left/right-most item will add an additional WindowPadding to reflect "
-            "on reaching the edge of the list.\n\nBecause the clipping rectangle of most window hides half "
-            "worth of WindowPadding on the left/right, using SetScrollFromPosX(+1) will usually result in "
-            "clipped text whereas the equivalent SetScrollFromPosY(+1) wouldn't.");
+            "Because the clipping rectangle of most window hides half worth of WindowPadding on the "
+            "left/right, using SetScrollFromPosX(+1) will usually result in clipped text whereas the "
+            "equivalent SetScrollFromPosY(+1) wouldn't.");
         ImGui::PushID("##HorizontalScrolling");
         for (int i = 0; i < 5; i++)
         {