Windows: use relative mouse movement for border resize when the border geometry has moved. (#1710)

(e.g. resizing a child window triggering parent scroll) to avoid resizing feedback loop.

docs/CHANGELOG.txt

@@ -96,6 +96,9 @@ Other changes:
     to false when popup is closed in ways other than clicking the close button. (#6900)
   - Double-clicking lower-left resize grip auto-resize (like lower-rightone).
   - Double-clicking bottom or right window border auto-resize on a singles axis.
+  - Use relative mouse movement for border resize when the border geometry has moved
+    (e.g. resizing a child window triggering parent scroll) in order to avoid resizing
+    feedback loops. Unless manually mouse-wheeling while border resizing. (#1710)
 - Separators:
   - Altered end-points to use more standard boundaries. (#205, #4787, #1643)
     Left position is always current cursor X position, right position is always work-rect

imgui.cpp

@@ -5964,7 +5964,7 @@ static int ImGui::UpdateWindowManualResize(ImGuiWindow* window, const ImVec2& si
         ImGuiID border_id = window->GetID(border_n + 4); // == GetWindowResizeBorderID()
         ItemAdd(border_rect, border_id, NULL, ImGuiItemFlags_NoNav);
         ButtonBehavior(border_rect, border_id, &hovered, &held, ImGuiButtonFlags_FlattenChildren | ImGuiButtonFlags_NoNavFocus);
-        //GetForegroundDrawLists(window)->AddRect(border_rect.Min, border_rect.Max, IM_COL32(255, 255, 0, 255));
+        //GetForegroundDrawList(window)->AddRect(border_rect.Min, border_rect.Max, IM_COL32(255, 255, 0, 255));
         if (hovered && g.HoveredIdTimer <= WINDOWS_RESIZE_FROM_EDGES_FEEDBACK_TIMER)
             hovered = false;
         if (hovered || held)
@@ -5983,13 +5983,43 @@ static int ImGui::UpdateWindowManualResize(ImGuiWindow* window, const ImVec2& si
         }
         else if (held)
         {
+            // Switch to relative resizing mode when border geometry moved (e.g. resizing a child altering parent scroll), in order to avoid resizing feedback loop.
+            // Currently only using relative mode on resizable child windows, as the problem to solve is more likely noticeable for them, but could apply for all windows eventually.
+            // FIXME: May want to generalize this idiom at lower-level, so more widgets can use it!
+            const bool just_scrolled_manually_while_resizing = (g.WheelingWindow != NULL && g.WheelingWindowScrolledFrame == g.FrameCount && IsWindowChildOf(window, g.WheelingWindow, false));
+            if (g.ActiveIdIsJustActivated || just_scrolled_manually_while_resizing)
+            {
+                g.WindowResizeBorderExpectedRect = border_rect;
+                g.WindowResizeRelativeMode = false;
+            }
+            if ((window->Flags & ImGuiWindowFlags_ChildWindow) && memcmp(&g.WindowResizeBorderExpectedRect, &border_rect, sizeof(ImRect)) != 0)
+                g.WindowResizeRelativeMode = true;
+
+            const ImVec2 border_curr = (window->Pos + ImMin(def.SegmentN1, def.SegmentN2) * window->Size);
+            const float border_target_rel_mode_for_axis = border_curr[axis] + g.IO.MouseDelta[axis];
+            const float border_target_abs_mode_for_axis = g.IO.MousePos[axis] - g.ActiveIdClickOffset[axis] + WINDOWS_HOVER_PADDING; // Match ButtonBehavior() padding above.
+
+            // Use absolute mode position
+            ImVec2 border_target = window->Pos;
+            border_target[axis] = border_target_abs_mode_for_axis;
+
+            // Use relative mode target for child window, ignore resize when moving back toward the ideal absolute position.
+            bool ignore_resize = false;
+            if (g.WindowResizeRelativeMode)
+            {
+                //GetForegroundDrawList()->AddText(GetMainViewport()->WorkPos, IM_COL32_WHITE, "Relative Mode");
+                border_target[axis] = border_target_rel_mode_for_axis;
+                if (g.IO.MouseDelta[axis] == 0.0f || (g.IO.MouseDelta[axis] > 0.0f) == (border_target_rel_mode_for_axis > border_target_abs_mode_for_axis))
+                    ignore_resize = true;
+            }
+
+            // Clamp, apply
             ImVec2 clamp_min(border_n == ImGuiDir_Right ? clamp_rect.Min.x : -FLT_MAX, border_n == ImGuiDir_Down || (border_n == ImGuiDir_Up && window_move_from_title_bar) ? clamp_rect.Min.y : -FLT_MAX);
             ImVec2 clamp_max(border_n == ImGuiDir_Left ? clamp_rect.Max.x : +FLT_MAX, border_n == ImGuiDir_Up ? clamp_rect.Max.y : +FLT_MAX);
-            ImVec2 border_target = window->Pos;
-            border_target[axis] = g.IO.MousePos[axis] - g.ActiveIdClickOffset[axis] + WINDOWS_HOVER_PADDING;
             border_target = ImClamp(border_target, clamp_min, clamp_max);
             if (window->Flags & ImGuiWindowFlags_ChildWindow) // Clamp resizing of childs within parent
                 border_target = ImClamp(border_target, window->ParentWindow->InnerClipRect.Min, window->ParentWindow->InnerClipRect.Max);
+            if (!ignore_resize)
                 CalcResizePosSizeFromAnyCorner(window, border_target, ImMin(def.SegmentN1, def.SegmentN2), &pos_target, &size_target);
         }
         if (hovered)
@@ -6043,6 +6073,10 @@ static int ImGui::UpdateWindowManualResize(ImGuiWindow* window, const ImVec2& si
     if (size_target.x != FLT_MAX || size_target.y != FLT_MAX || pos_target.x != FLT_MAX || pos_target.y != FLT_MAX)
         MarkIniSettingsDirty(window);
 
+    // Recalculate next expected border expected coordinates
+    if (*border_held != -1)
+        g.WindowResizeBorderExpectedRect = GetResizeBorderRect(window, *border_held, grip_hover_inner_size, WINDOWS_HOVER_PADDING);
+
     return ret_auto_fit_mask;
 }
 
@@ -8931,6 +8965,7 @@ void ImGui::UpdateMouseWheel()
                 float max_step = window->InnerRect.GetWidth() * 0.67f;
                 float scroll_step = ImTrunc(ImMin(2 * window->CalcFontSize(), max_step));
                 SetScrollX(window, window->Scroll.x - wheel.x * scroll_step);
+                g.WheelingWindowScrolledFrame = g.FrameCount;
             }
             if (do_scroll[ImGuiAxis_Y])
             {
@@ -8938,6 +8973,7 @@ void ImGui::UpdateMouseWheel()
                 float max_step = window->InnerRect.GetHeight() * 0.67f;
                 float scroll_step = ImTrunc(ImMin(5 * window->CalcFontSize(), max_step));
                 SetScrollY(window, window->Scroll.y - wheel.y * scroll_step);
+                g.WheelingWindowScrolledFrame = g.FrameCount;
             }
         }
 }

imgui_internal.h

@@ -1899,6 +1899,7 @@ struct ImGuiContext
     ImGuiWindow*            WheelingWindow;                     // Track the window we started mouse-wheeling on. Until a timer elapse or mouse has moved, generally keep scrolling the same window even if during the course of scrolling the mouse ends up hovering a child window.
     ImVec2                  WheelingWindowRefMousePos;
     int                     WheelingWindowStartFrame;           // This may be set one frame before WheelingWindow is != NULL
+    int                     WheelingWindowScrolledFrame;
     float                   WheelingWindowReleaseTimer;
     ImVec2                  WheelingWindowWheelRemainder;
     ImVec2                  WheelingAxisAvg;
@@ -2091,6 +2092,8 @@ struct ImGuiContext
     ImU32                   ColorEditSavedColor;                // RGB value with alpha set to 0.
     ImVec4                  ColorPickerRef;                     // Initial/reference color at the time of opening the color picker.
     ImGuiComboPreviewData   ComboPreviewData;
+    ImRect                  WindowResizeBorderExpectedRect;     // Expected border rect, switch to relative edit if moving
+    bool                    WindowResizeRelativeMode;
     float                   SliderGrabClickOffset;
     float                   SliderCurrentAccum;                 // Accumulated slider delta when using navigation controls.
     bool                    SliderCurrentAccumDirty;            // Has the accumulated slider delta changed since last time we tried to apply it?
@@ -2187,7 +2190,7 @@ struct ImGuiContext
         HoveredWindowUnderMovingWindow = NULL;
         MovingWindow = NULL;
         WheelingWindow = NULL;
-        WheelingWindowStartFrame = -1;
+        WheelingWindowStartFrame = WheelingWindowScrolledFrame = -1;
         WheelingWindowReleaseTimer = 0.0f;
 
         DebugHookIdInfo = 0;
@@ -2289,6 +2292,7 @@ struct ImGuiContext
         ColorEditCurrentID = ColorEditSavedID = 0;
         ColorEditSavedHue = ColorEditSavedSat = 0.0f;
         ColorEditSavedColor = 0;
+        WindowResizeRelativeMode = false;
         SliderGrabClickOffset = 0.0f;
         SliderCurrentAccum = 0.0f;
         SliderCurrentAccumDirty = false;