Nav: Fixed rare crash when e.g. releasing Alt-key while focusing a wi…

…ndow with a menu at the same frame as clearing the focus. This was in most noticeable in some back-ends with emits key release events when focusing another viewport. (#2609)

docs/CHANGELOG.txt

@@ -56,6 +56,10 @@ Other Changes:
 - Combo: Fixed rounding not applying with the ImGuiComboFlags_NoArrowButton flag. (#2607) [@DucaRii]
 - Nav: Fixed gamepad/keyboard moving of window affecting contents size incorrectly, sometimes leading
   to scrollbars appearing during the movement.
+- Nav: Fixed rare crash when e.g. releasing Alt-key while focusing a window with a menu at the same
+  frame as clearing the focus. This was in most noticeable in back-ends such as Glfw and SDL which
+  emits key release events when focusing another viewport, leading to Alt+clicking on void on another
+  viewport triggering the issue. (#2609)
 - Style: Added style.WindowMenuButtonPosition (left/right, defaults to ImGuiDir_Left) to move the
   collapsing/docking button to the other side of the title bar.
 - Style: Made window close button cross slightly smaller.

imgui.cpp

@@ -7933,10 +7933,10 @@ static void ImGui::NavUpdate()
         g.IO.NavInputsDownDuration[i] = (g.IO.NavInputs[i] > 0.0f) ? (g.IO.NavInputsDownDuration[i] < 0.0f ? 0.0f : g.IO.NavInputsDownDuration[i] + g.IO.DeltaTime) : -1.0f;
 
     // Process navigation init request (select first/default focus)
-    if (g.NavInitResultId != 0 && (!g.NavDisableHighlight || g.NavInitRequestFromMove))
+    // In very rare cases g.NavWindow may be null (e.g. clearing focus after requesting an init request, which does happen when releasing Alt while clicking on void)
+    if (g.NavInitResultId != 0 && (!g.NavDisableHighlight || g.NavInitRequestFromMove) && g.NavWindow)
     {
         // Apply result from previous navigation init request (will typically select the first item, unless SetItemDefaultFocus() has been called)
-        IM_ASSERT(g.NavWindow);
         if (g.NavInitRequestFromMove)
             SetNavIDWithRectRel(g.NavInitResultId, g.NavLayer, g.NavInitResultRectRel);
         else