Finite (but large) scrollback (not infinite)

[yumeyao]

Description of the new feature

Currently the maximum historysize is 32767, which is really a small number and quite insufficient. There is also another feature request #1410 to set the line limit to infinity.

This is not a duplicate of that issue because increasing the limit only requires a small code change to just increase the buffer limit without losing performance, whereas infinity requires more complicated code changes to make the buffer able to resize/extend (maybe using a rope).

I'm saying without losing performance because #15524 was merged, now it's using virtual memory allocations so that the "real" memory use increases only when you really use that much buffer.

And the code change is quite small, just change these Utils::ClampToShortMax and gsl::narrow<uint16_t>.

// src/cascadia/TerminalCore/Terminal.cpp:
// void Terminal::Create(til::size viewportSize, til::CoordType scrollbackLines, Renderer& renderer)
    const til::size bufferSize{ viewportSize.width,
                                Utils::ClampToShortMax(viewportSize.height + scrollbackLines, 1) };

// void Terminal::CreateFromSettings(ICoreSettings settings, Renderer& renderer)
    Create(viewportSize, Utils::ClampToShortMax(settings.HistorySize(), 0), renderer);

// src/buffer/out/textBuffer.cpp
// void TextBuffer::_reserve(til::size screenBufferSize, const TextAttribute& defaultAttributes)
    const auto h = gsl::narrow<uint16_t>(screenBufferSize.height);

// src/buffer/out/textBuffer.hpp
// class TextBuffer final
    uint16_t _height = 0;

Currently these code effectively limits the max buffer size(height) to 32767, we can just increase the limit here.

Proposed technical implementation details

• We can just change the hard limit in TextBuffer to int32_t (2^31-1) because CoordType is currently int32_t and it is large enough.
  • We can change the code above in textBuffer.hpp|cpp to use CoordType or int32_t
  • We can also change type of _width for alignment. Also int16_t is slow on modern x86(ia32/x64)/ARM64 CPUs (although it's not the key point here but nice to have)

    uint16_t _width = 0;
    uint16_t _height = 0;
    

• Relax the hard limit when creating the Terminal class
  • maybe clamps to INT_MAX
• Better to have another layer of soft limit in settings
  • maybe we don't need INT_MAX, we can really set it to something like 999'999'999 or 100'000'000. The reason here is because I think many people (like me) hates the 32767 limit (but not aware there is a hard limit inside) and fill tons of 9s in the settings. This helps reduce the virtual allocation size because virtual address space is still a resource and wasting it too much may have a side-effect that limits down the maximum tabs a user can open.
  • After the change we can let users know there was a hard limit and now it's increased, please revise your settings of this value blah blah, so that if a user previously really set it to more than 10 9s and clamps to INT_MAX, the user can set it to a more reasonable value to avoid potential issues like stated above.

[lhecker]

A "small code change" is it? 😏 See #18290.