Made mouse drag distance calculation more robust to invalid mouse position. (#1845)

imgui.cpp

@@ -3418,7 +3418,8 @@ static void ImGui::UpdateMouseInputs()
         {
             if (g.Time - g.IO.MouseClickedTime[i] < g.IO.MouseDoubleClickTime)
             {
-                if (ImLengthSqr(g.IO.MousePos - g.IO.MouseClickedPos[i]) < g.IO.MouseDoubleClickMaxDist * g.IO.MouseDoubleClickMaxDist)
+                ImVec2 delta_from_click_pos = IsMousePosValid(&g.IO.MousePos) ? (g.IO.MousePos - g.IO.MouseClickedPos[i]) : ImVec2(0.0f, 0.0f);
+                if (ImLengthSqr(delta_from_click_pos) < g.IO.MouseDoubleClickMaxDist * g.IO.MouseDoubleClickMaxDist)
                     g.IO.MouseDoubleClicked[i] = true;
                 g.IO.MouseClickedTime[i] = -FLT_MAX;    // so the third click isn't turned into a double-click
             }
@@ -3432,10 +3433,11 @@ static void ImGui::UpdateMouseInputs()
         }
         else if (g.IO.MouseDown[i])
         {
-            ImVec2 mouse_delta = g.IO.MousePos - g.IO.MouseClickedPos[i];
-            g.IO.MouseDragMaxDistanceAbs[i].x = ImMax(g.IO.MouseDragMaxDistanceAbs[i].x, mouse_delta.x < 0.0f ? -mouse_delta.x : mouse_delta.x);
-            g.IO.MouseDragMaxDistanceAbs[i].y = ImMax(g.IO.MouseDragMaxDistanceAbs[i].y, mouse_delta.y < 0.0f ? -mouse_delta.y : mouse_delta.y);
-            g.IO.MouseDragMaxDistanceSqr[i] = ImMax(g.IO.MouseDragMaxDistanceSqr[i], ImLengthSqr(mouse_delta));
+            // Maintain the maximum distance we reaching from the initial click position, which is used with dragging threshold
+            ImVec2 delta_from_click_pos = IsMousePosValid(&g.IO.MousePos) ? (g.IO.MousePos - g.IO.MouseClickedPos[i]) : ImVec2(0.0f, 0.0f);
+            g.IO.MouseDragMaxDistanceSqr[i] = ImMax(g.IO.MouseDragMaxDistanceSqr[i], ImLengthSqr(delta_from_click_pos));
+            g.IO.MouseDragMaxDistanceAbs[i].x = ImMax(g.IO.MouseDragMaxDistanceAbs[i].x, delta_from_click_pos.x < 0.0f ? -delta_from_click_pos.x : delta_from_click_pos.x);
+            g.IO.MouseDragMaxDistanceAbs[i].y = ImMax(g.IO.MouseDragMaxDistanceAbs[i].y, delta_from_click_pos.y < 0.0f ? -delta_from_click_pos.y : delta_from_click_pos.y);
         }
         if (g.IO.MouseClicked[i]) // Clicking any mouse button reactivate mouse hovering which may have been deactivated by gamepad/keyboard navigation
             g.NavDisableMouseHover = false;