fix per-file checkered background not working for PDF documents

The checkered pattern and kColorUnset both used the same sentinel value,
so Canvas.cpp couldn't distinguish "no per-file color" from "per-file
checkered". Added bgColorCheckered flag to WindowTab to track this
explicitly. Also fixed the no-gradient branch to use colDocBg instead
of ThemeMainWindowBackgroundColor().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kjk

src/Canvas.cpp

@@ -1484,7 +1484,9 @@ static bool DrawDocument(MainWindow* win, HDC hdc, RECT* rcArea) {
 
     // per-document background color from FileState overrides everything
     WindowTab* curTab = win->CurrentTab();
-    if (curTab && curTab->bgColor != kColorUnset) {
+    if (curTab && curTab->bgColorCheckered) {
+        colDocBg = kColorUnset;
+    } else if (curTab && curTab->bgColor != kColorUnset) {
         colDocBg = curTab->bgColor;
     }
 
@@ -1505,8 +1507,7 @@ static bool DrawDocument(MainWindow* win, HDC hdc, RECT* rcArea) {
     } else if (colDocBg == kColorUnset) {
         paintBgOrCheckerboard(colDocBg, rcArea);
     } else if (0 == nGCols) {
-        auto col = ThemeMainWindowBackgroundColor();
-        AutoDeleteBrush brush = CreateSolidBrush(col);
+        AutoDeleteBrush brush = CreateSolidBrush(colDocBg);
         FillRect(hdc, rcArea, brush);
     } else {
         COLORREF colors[3];

src/SumatraPDF.cpp

@@ -1426,6 +1426,7 @@ static void ReplaceDocumentInCurrentTab(LoadArgs* args, DocController* ctrl, Fil
         ParsedColor* bgParsed = GetPrefsColor(fs->bgCol);
         if (bgParsed->parsedOk) {
             tab->bgColor = bgParsed->col;
+            tab->bgColorCheckered = (bgParsed->col == kColorUnset);
         }
         ParsedColor* tabColParsed = GetPrefsColor(fs->tabCol);
         if (tabColParsed->parsedOk) {
@@ -4438,6 +4439,7 @@ static void OnMenuChangeBackgroundColor(MainWindow* win) {
             str::ReplaceWithCopy(&fs->bgCol, "");
         }
         tab->bgColor = kColorUnset;
+        tab->bgColorCheckered = false;
         SaveSettings();
     } else {
         // apply to this file only
@@ -4447,6 +4449,7 @@ static void OnMenuChangeBackgroundColor(MainWindow* win) {
             fs->bgColParsed.wasParsed = false;
         }
         tab->bgColor = newColor;
+        tab->bgColorCheckered = result.isCheckered;
         SaveSettings();
     }
     // trigger repaint

src/WindowTab.h

@@ -57,6 +57,8 @@ struct WindowTab {
 
     // per-document background color from FileState; kColorUnset = use default
     COLORREF bgColor = kColorUnset;
+    // true if per-document background is explicitly set to checkered pattern
+    bool bgColorCheckered = false;
     // per-document tab color from FileState; kColorUnset = use default
     COLORREF tabColor = kColorUnset;