Bug 1501665 - No bug: Add comment with rationale behind choice of default viewport. r=dbaron

Especially in view of the patches for bug 1501665, which seem to have some-
what misunderstood the reason for the choice of viewport width here.

Differential Revision: https://phabricator.services.mozilla.com/D20950

--HG--
extra : moz-landing-system : lando

dom/base/Document.cpp

@@ -6792,8 +6792,16 @@ nsViewportInfo Document::GetViewportInfo(const ScreenIntSize& aDisplaySize) {
       // https://drafts.csswg.org/css-device-adapt/#resolve-width
       if (width == nsViewportInfo::Auto) {
         if (height == nsViewportInfo::Auto || aDisplaySize.height == 0) {
-          // Stretch CSS pixel size of viewport to keep device pixel size
-          // unchanged after full zoom applied.
+          // If we don't have any applicable viewport width constraints, this is
+          // most likely a desktop page written without mobile devices in mind.
+          // We use the desktop mode viewport for those pages by default,
+          // because a narrow viewport based on typical mobile device screen
+          // sizes (especially in portrait mode) will frequently break the
+          // layout of such pages. To keep text readable in that case, we rely
+          // on font inflation instead.
+
+          // Divide by fullZoom to stretch CSS pixel size of viewport in order
+          // to keep device pixel size unchanged after full zoom applied.
           // See bug 974242.
           width = gfxPrefs::DesktopViewportWidth() / fullZoom;
         } else {