Bug 1765059: Stop storing/reading the print resolution in about:config pref. r=bobowen

This per-printer "print_resolution" about:config pref (which ends up in
nsPrintSettings 'mResolution' member-var) is never exposed in UI, and it's
almost entirely unused.  For the usages that do exist (usages of
nsPrintSettings::mResolution), we potentially do the wrong thing when the value
comes from an about:config pref, as described in this bug.  These usages scale
the printed output *as if we're printing to a printer with the given
resolution*, though of course we have no guarantee of this being the printer's
actual resolution, when this value comes from about:config prefs.

After this patch, nsPrintSettings will simply use the value that we actually
get from the printer, instead of this potentially-bogus value from
about:config.  This will avoid the scaling issues described in this bug.

Differential Revision: https://phabricator.services.mozilla.com/D146261

widget/nsIPrintSettings.idl

@@ -42,7 +42,7 @@ interface nsIPrintSettings : nsISupports
   const unsigned long kInitSaveBGColors       = 0x00000080;
   const unsigned long kInitSaveBGImages       = 0x00000100;
   const unsigned long kInitSavePaperSize      = 0x00000200;
-  const unsigned long kInitSaveResolution     = 0x00000400;
+  /* Flag 0x00000400 is unused */
   const unsigned long kInitSaveDuplex         = 0x00000800;
   /* Flag 0x00001000 is unused */
   /* Flag 0x00002000 is unused */

widget/nsPrintSettingsService.cpp

@@ -84,7 +84,6 @@ static const char kPrintBGColors[] = "print_bgcolor";
 static const char kPrintBGImages[] = "print_bgimages";
 static const char kPrintShrinkToFit[] = "print_shrink_to_fit";
 static const char kPrintScaling[] = "print_scaling";
-static const char kPrintResolution[] = "print_resolution";
 static const char kPrintDuplex[] = "print_duplex";
 
 static const char kJustLeft[] = "left";
@@ -570,15 +569,6 @@ nsresult nsPrintSettingsService::ReadPrefs(nsIPrintSettings* aPS,
     }
   }
 
-  if (aFlags & nsIPrintSettings::kInitSaveResolution) {
-    // DPI. Again, an arbitrary range mainly to purge bad values that have made
-    // their way into user prefs.
-    if (GETINTPREF(kPrintResolution, &iVal) && iVal >= 50 && iVal <= 12000) {
-      aPS->SetResolution(iVal);
-      noValidPrefsFound = false;
-    }
-  }
-
   if (aFlags & nsIPrintSettings::kInitSaveDuplex) {
     if (GETINTPREF(kPrintDuplex, &iVal)) {
       aPS->SetDuplex(iVal);
@@ -588,6 +578,7 @@ nsresult nsPrintSettingsService::ReadPrefs(nsIPrintSettings* aPS,
 
   // Not Reading In:
   //   Number of Copies
+  //   Print Resolution
 
   return noValidPrefsFound ? NS_ERROR_NOT_AVAILABLE : NS_OK;
 }
@@ -769,12 +760,6 @@ nsresult nsPrintSettingsService::WritePrefs(nsIPrintSettings* aPS,
     }
   }
 
-  if (aFlags & nsIPrintSettings::kInitSaveResolution) {
-    if (NS_SUCCEEDED(aPS->GetResolution(&iVal))) {
-      Preferences::SetInt(GetPrefName(kPrintResolution, aPrinterName), iVal);
-    }
-  }
-
   if (aFlags & nsIPrintSettings::kInitSaveDuplex) {
     if (NS_SUCCEEDED(aPS->GetDuplex(&iVal))) {
       Preferences::SetInt(GetPrefName(kPrintDuplex, aPrinterName), iVal);
@@ -783,6 +768,7 @@ nsresult nsPrintSettingsService::WritePrefs(nsIPrintSettings* aPS,
 
   // Not Writing Out:
   //   Number of Copies
+  //   Print Resolution
 
   return NS_OK;
 }