The mess that is the VCL

 Let me count the ways, in no particular order and in no way exhaustive:

• OutputDevice is the base class for printing, windowing and PDFs. It doesn't just do output. 
• OutputDevice has GetOutDevType() because the base class needs to know what child class is using it. Ugh. 
• OutputDevice drawing primitives not only draw, but they record a metafile. There are literally functions that turn off drawing and just let it record the metafile. I made an attempt at seperating the concerns, but it got nowhere. 
• VCL relies on DrawingLayer and DrawingLayer relies on the VCL. 
• There is a concept of a VirtualDevice, which is derived from OutputDevice. VirtualDevice does a bunch of things, but one of which is alpha-handling. In OutputDevice, there is a member which is a VirtualDevice. Each drawing function in Outputdevice calls upon the correlated drawing function in this member VirtualDevice.
  
• Bitmaps don't get modified via the Bitmap class. Instead, you have to use BitmapInfoAccess, BitmapReadAccess and BitmapWriteAccess. I'm still puzzling out why these are seperate classes. 
• Bitmaps are transformed in SalGraphics indirectly via OutputDevice. Except when they aren't, in which case it fails, whereby OutputDevice tries an alternative way via SalGraphics. Otherwise, it tries its own poor man approach at drawing the bitmap. Consequently, often times you bypass the platform optimized ways of doing things, because its not been implemented.
  
• Fonts are lazy loaded from OutputDevice. There is no central font manager. To get the fonts, you have to go through SalGraphics. To get a SalGraphics, you need to initialize a lot of stuff not related to fonts. 
• Font caching is done from OutputDevice. Lazily. Font data is updated for all frames. Frames are a concept needed for Windows. Frames are not a concept needed by Printers and VirtualDevices, or even PDFs. Note that Printers, VirtualDevices and PDFs all inherit from OutputDevice. 
  
• OutputDevice converts between "logical" units and display units. It's a nightmare to know what each function needs what sort of units. For the mapping between units, I refer you to vcl/source/gdi/mapmod.cxx and vcl/source/outdev/map.cxx
• There is tools and basegfx. They do the same thing, though basegfx is considerably better written. You have Size and B2DSize, Point and B2DPoint, Polygon and B2DPolygon, PolyPolygon and B2DPolyPolygon. OutputDevice must handle it all. 
• Gradient handling is sort of half baked in OutputDevice, much of gradient handling is done in other modules. 
  
• Font substitution is truly, truly weird. PhysicalFontSelect::FindFontFamilyByAttributes() has clearly got a bug in it - (e.g. ImplFontAttrs::None == ((nSearchType ^ nMatchType) & ImplFontAttrs::Rounded an XOR?) and it is a truly strange weighting scheme. Yes, I did try to untangle that beast with proper unit tests, but gave up after being told I was being unreasonable. 
• There is VCL, canvas, cppcanvas and drawinglayer. drawinglayer is way better than VCL, but we are stuck with VCL for everything. 
• Consider the following Window hierarchy: WorkWindow inherits from SystemWindow, which inherits from Window. Window holds an OutputDevice to do stuff. WindowOutputDevice derives from OutputDevice. This is needed because OutputDevice often needs to know if it is doing Window operations, via WindowOutputDevice. Try untangling this in your head.
  
• Text layout is its own beast, and has its own set of classes. A lot of text layout is worked out in OutputDevice. 
  
• Text layout is done via OutputDevice::ImplLayout(). I present to you the ImplLayout function signature:
  
  

      std::unique_ptr<SalLayout> ImplLayout(
          const OUString&, sal_Int32 nIndex, sal_Int32 nLen, const Point& rLogicPos = Point(0, 0),
          tools::Long nLogicWidth = 0, KernArraySpan aKernArray = KernArraySpan(),
          std::span<const sal_Bool> pKashidaArray = {}, SalLayoutFlags flags = SalLayoutFlags::NONE,
          vcl::text::TextLayoutCache const* = nullptr, const SalLayoutGlyphs* pGlyphs = nullptr,
          std::optional<sal_Int32> nDrawOriginCluster = std::nullopt,
          std::optional<sal_Int32> nDrawMinCharPos = std::nullopt,
          std::optional<sal_Int32> nDrawEndCharPos = std::nullopt) const;