Mask viewport split dividers under floating windows

The divider hit-zones are raw screen rects; gate them on
IsWindowHovered(ChildWindows) so a window stacked over the viewport
(e.g. the Material Editor) doesn't highlight or start a resize drag
through it. In-progress drags still track until release.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -78,3 +78,4 @@ imgui.ini
 
 # OCIO config LUT files — downloaded automatically at CMake configure time
 resources/OpenColorIO-Configs/
+resources/hdri/

src/ui/ViewportPanel.cpp

@@ -247,6 +247,12 @@ bool ViewportPanel::IsMouseOverDivider(ImVec2 origin, ImVec2 total) const
     if (m_maximizedTileIndex >= 0)         return false;
     if (m_draggingDivH || m_draggingDivV)  return true;
 
+    // The hit-zones below are raw screen rects; without this gate a floating
+    // window stacked over the viewport (e.g. the Material Editor) wouldn't
+    // mask them and the divider would still react underneath it.
+    if (!ImGui::IsWindowHovered(ImGuiHoveredFlags_ChildWindows))
+        return false;
+
     const float kDivHalf = 3.0f;
     ImVec2 mouse = ImGui::GetMousePos();
 
@@ -283,6 +289,11 @@ void ViewportPanel::DrawDividers(ImVec2 origin, ImVec2 total)
     // accidentally triggered when the mouse drifts over the hit-zone.
     bool        lmbClicked  = ImGui::IsMouseClicked(ImGuiMouseButton_Left);
     bool        lmbReleased = ImGui::IsMouseReleased(ImGuiMouseButton_Left);
+    // Raw screen-rect hit-testing below — require this window (or its tiles)
+    // to actually be the hovered window, so a floating window stacked on top
+    // (e.g. the Material Editor) masks the dividers. In-progress drags are
+    // exempt: they must keep tracking until LMB release wherever the mouse is.
+    bool        windowHovered = ImGui::IsWindowHovered(ImGuiHoveredFlags_ChildWindows);
 
     // Vertical divider (HSplit / Quad)
     if (m_layout == LayoutMode::HSplit || m_layout == LayoutMode::Quad) {
@@ -290,7 +301,7 @@ void ViewportPanel::DrawDividers(ImVec2 origin, ImVec2 total)
         ImVec2 hMin(divX - kDivThick * 0.5f, origin.y);
         ImVec2 hMax(divX + kDivThick * 0.5f, origin.y + total.y);
 
-        bool hovering = !m_draggingDivV &&
+        bool hovering = windowHovered && !m_draggingDivV &&
                         mousePos.x >= hMin.x && mousePos.x <= hMax.x &&
                         mousePos.y >= hMin.y && mousePos.y <= hMax.y;
 
@@ -315,7 +326,7 @@ void ViewportPanel::DrawDividers(ImVec2 origin, ImVec2 total)
         ImVec2 hMin(origin.x,           divY - kDivThick * 0.5f);
         ImVec2 hMax(origin.x + total.x, divY + kDivThick * 0.5f);
 
-        bool hovering = !m_draggingDivH &&
+        bool hovering = windowHovered && !m_draggingDivH &&
                         mousePos.x >= hMin.x && mousePos.x <= hMax.x &&
                         mousePos.y >= hMin.y && mousePos.y <= hMax.y;