Docking: comments, clairfy why we store style override on dragged win…

…dows. Clarify why we don't need to store alpha. (#7535, #2771)

Amend ebbb98d

imgui.cpp

@@ -19031,6 +19031,13 @@ static ImGuiDockNode* ImGui::DockContextBindNodeToWindow(ImGuiContext* ctx, ImGu
     return node;
 }
 
+static void StoreDockStyleForWindow(ImGuiWindow* window)
+{
+    ImGuiContext& g = *GImGui;
+    for (int color_n = 0; color_n < ImGuiWindowDockStyleCol_COUNT; color_n++)
+        window->DockStyle.Colors[color_n] = ImGui::ColorConvertFloat4ToU32(g.Style.Colors[GWindowDockStyleColors[color_n]]);
+}
+
 void ImGui::BeginDocked(ImGuiWindow* window, bool* p_open)
 {
     ImGuiContext& g = *GImGui;
@@ -19094,8 +19101,7 @@ void ImGui::BeginDocked(ImGuiWindow* window, bool* p_open)
     }
 
     // Store style overrides
-    for (int color_n = 0; color_n < ImGuiWindowDockStyleCol_COUNT; color_n++)
-        window->DockStyle.Colors[color_n] = ColorConvertFloat4ToU32(g.Style.Colors[GWindowDockStyleColors[color_n]]);
+    StoreDockStyleForWindow(window);
 
     // Fast path return. It is common for windows to hold on a persistent DockId but be the only visible window,
     // and never create neither a host window neither a tab bar.
@@ -19185,10 +19191,7 @@ void ImGui::BeginDockableDragDropSource(ImGuiWindow* window)
     {
         SetDragDropPayload(IMGUI_PAYLOAD_TYPE_WINDOW, &window, sizeof(window));
         EndDragDropSource();
-
-        // Store style overrides
-        for (int color_n = 0; color_n < ImGuiWindowDockStyleCol_COUNT; color_n++)
-            window->DockStyle.Colors[color_n] = ColorConvertFloat4ToU32(g.Style.Colors[GWindowDockStyleColors[color_n]]);
+        StoreDockStyleForWindow(window); // Store style overrides while dragging (even when not docked) because docking preview may need it.
     }
 }
 

imgui_internal.h

@@ -161,6 +161,7 @@ struct ImGuiTableColumnsSettings;   // Storage for a column .ini settings
 struct ImGuiTypingSelectState;      // Storage for GetTypingSelectRequest()
 struct ImGuiTypingSelectRequest;    // Storage for GetTypingSelectRequest() (aimed to be public)
 struct ImGuiWindow;                 // Storage for one window
+struct ImGuiWindowDockStyle;        // Storage for window-style data which needs to be stored for docking purpose
 struct ImGuiWindowTempData;         // Temporary storage for one window (that's the data which in theory we could ditch at the end of the frame, in practice we currently keep it for each window)
 struct ImGuiWindowSettings;         // Storage for a window .ini settings (we keep one of those even if the actual window wasn't instanced during this session)
 
@@ -1872,6 +1873,7 @@ enum ImGuiWindowDockStyleCol
     ImGuiWindowDockStyleCol_COUNT
 };
 
+// We don't store style.Alpha: dock_node->LastBgColor embeds it and otherwise it would only affect the docking tab, which intuitively I would say we don't want to.
 struct ImGuiWindowDockStyle
 {
     ImU32 Colors[ImGuiWindowDockStyleCol_COUNT];