Viewport: Shuffled code in Begin to reset size of reappearing popups prior to selecting viewport, to avoid mistakenly using an old size. Moved code out of UpdateSelectWindowViewport() to the section of Begin where positions is being locked down, to avoid code in-between duplicating this. (#1542)

imgui.cpp

@@ -6250,17 +6250,6 @@ static void ImGui::UpdateSelectWindowViewport(ImGuiWindow* window)
     if (window->ViewportOwned && !(window->Viewport->Flags & ImGuiViewportFlags_NoDecoration))
         window->Flags |= ImGuiWindowFlags_NoTitleBar;
 
-    if (window->ViewportOwned)
-    {
-        // We currently have window fully covering a viewport and we disable WindowBg alpha, so clearing is not necessary
-        window->Viewport->Flags |= ImGuiViewportFlags_NoRendererClear;
-
-        // Position
-        SetWindowPos(window, window->Viewport->Pos, ImGuiCond_Always);
-        if (window->Viewport->PlatformRequestResize)
-            SetWindowSize(window, window->Viewport->Size, ImGuiCond_Always);
-    }
-
     window->ViewportId = window->Viewport->ID;
 }
 
@@ -6550,8 +6539,34 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         window->LastFrameActive = current_frame;
         window->IDStack.resize(1);
 
-        // VIEWPORT
-        // We need to do this before using any style/font sizes, as viewport with a different DPI will affect those sizes.
+        // UPDATE CONTENTS SIZE, UPDATE HIDDEN STATUS
+
+        // Update contents size from last frame for auto-fitting (or use explicit size)
+        window->SizeContents = CalcSizeContents(window);
+        if (window->HiddenFrames > 0)
+            window->HiddenFrames--;
+
+        // Hide new windows for one frame until they calculate their size
+        if (window_just_created && (!window_size_x_set_by_api || !window_size_y_set_by_api))
+            window->HiddenFrames = 1;
+
+        // Hide popup/tooltip window when re-opening while we measure size (because we recycle the windows)
+        // We reset Size/SizeContents for reappearing popups/tooltips early in this function, so further code won't be tempted to use the old size.
+        if (window_just_activated_by_user && (flags & (ImGuiWindowFlags_Popup | ImGuiWindowFlags_Tooltip)) != 0)
+        {
+            window->HiddenFrames = 1;
+            if (flags & ImGuiWindowFlags_AlwaysAutoResize)
+            {
+                if (!window_size_x_set_by_api)
+                    window->Size.x = window->SizeFull.x = 0.f;
+                if (!window_size_y_set_by_api)
+                    window->Size.y = window->SizeFull.y = 0.f;
+                window->SizeContents = ImVec2(0.f, 0.f);
+            }
+        }
+
+        // SELECT VIEWPORT
+        // We need to do this before using any style/font sizes, as viewport with a different DPI may affect font sizes.
 
         UpdateSelectWindowViewport(window);
         SetCurrentViewport(window->Viewport);
@@ -6586,31 +6601,6 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         }
         window->CollapseToggleWanted = false;
 
-        // UPDATE CONTENTS SIZE, REAPPEARING SIZE AND HIDDEN STATUS
-
-        // Update contents size from last frame for auto-fitting (unless explicitly specified)
-        window->SizeContents = CalcSizeContents(window);
-        if (window->HiddenFrames > 0)
-            window->HiddenFrames--;
-
-        // Hide new windows for one frame until they calculate their size
-        if (window_just_created && (!window_size_x_set_by_api || !window_size_y_set_by_api))
-            window->HiddenFrames = 1;
-
-        // Hide popup/tooltip window when re-opening while we measure size (because we recycle the windows)
-        if (window_just_activated_by_user && (flags & (ImGuiWindowFlags_Popup | ImGuiWindowFlags_Tooltip)) != 0)
-        {
-            window->HiddenFrames = 1;
-            if (flags & ImGuiWindowFlags_AlwaysAutoResize)
-            {
-                if (!window_size_x_set_by_api)
-                    window->Size.x = window->SizeFull.x = 0.f;
-                if (!window_size_y_set_by_api)
-                    window->Size.y = window->SizeFull.y = 0.f;
-                window->SizeContents = ImVec2(0.f, 0.f);
-            }
-        }
-
         // SIZE
 
         // Calculate auto-fit size, handle automatic resize
@@ -6694,6 +6684,17 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
                 window->PosFloat = ImMax(window->PosFloat + window->Size, viewport_rect.Min + padding) - window->Size;
                 window->PosFloat = ImMin(window->PosFloat, viewport_rect.Max - padding);
             }
+
+        // Position window to fit within viewport
+        // We can also tell the back-end that clearing the platform window won't be necessary, as our window is filling the viewport and we have disabled BgAlpha
+        if (window->ViewportOwned)
+        {
+            window->Viewport->Flags |= ImGuiViewportFlags_NoRendererClear;
+            window->PosFloat = window->Viewport->Pos;
+            if (window->Viewport->PlatformRequestResize)
+                window->Size = window->SizeFull = window->Viewport->Size;
+        }
+
         window->Pos = ImFloor(window->PosFloat);
 
         // Default item width. Make it proportional to window size if window manually resizes