Prevent panes from shrinking below 2x2 cells

[zhangxaochen]

Summary of the Pull Request

Raise the visible terminal minimum from 1×1 cells to 2×2 cells so a pane or window can no longer reach a 1-column viewport (keyboard, mouse, split-pane, wt --size, initialCols/initialRows, or VT resize).

This is a mitigation. It closes the hang for users, but does not fix TextBuffer::Reflow when a 2-column glyph is copied into a 1-column row. TextBuffer still accepts a 1-column size; Cascadia no longer asks for one.

Closes #19996.

References and Relevant Issues

• Terminal hangs when resizing pane to minimum width with Alt+Shift+Left #19996 (this bug). DHowett reproduced 2026-03-20:
  ROW::_uncheckedIsTrailer → ROW::_adjustForward → ROW::WriteHelper::Finish → ROW::CopyTextFrom → TextBuffer::Reflow
  ("resizing a 2-column glyph to a single column with reflow explodes the terminal")
• Not ConPTY hangs or enters infinite loop when writing a 2-column character to a 1-column wide terminal #19922 / Fix a deadlock during cooked reads #19994: that was ConPTY writing a 2-column character into an already 1-column terminal.
• Not Holding Alt+Shift+Left/Right to resize panes stalls the UI (4+ panes) #20562 / Throttle keyboard pane resize to avoid UI stalls #20563: that only throttles keyboard resizePane XAML layout. The splitter can still reach 1 cell, so the hang remains.
• j4james: 2×2 is the VT theoretical minimum (DECSTBM height ≥ 2, DECSLRM width ≥ 2).

Detailed Description of the Pull Request / Additional comments

Shared floor is MINIMUM_VISIBLE_CELLS (2) in DefaultSettings.h.

• TermControl::MinimumSize / GetProposedDimensions / GetNewDimensions
• ControlCore create and _refreshSizeUnderLock (the UserResize hang path)
• Terminal::Create / CreateFromSettings / UserResize
• HwndTerminal
• settings initialCols / initialRows clamp and the Launch page minimum

Pane::_GetMinSize / _ClampSplitPosition / _CalcSnappedDimension / PreCalculateCanSplit still consume MinimumSize(), so pane resize stays covered.

Left alone on purpose:

• TextBuffer / conhost 1×1 — remaining Reflow root-cause
• ConptyConnection / AppHost max(..., 1) — PTY size and pixel clamps, not cell counts

ASCII-only panes already survived a 1-cell clamp. Wide glyphs (CJK, emoji, starship prompts) did not.

Validation Steps Performed

• Confirmed the hang on unpatched Dev (WindowsTerminalDev) and Store 1.24.11911 with echo 音楽🎵测试 in view, then shrink the left pane to the stop.
• Live dump of hung Dev PID 27880: UI thread in TextBuffer::Reflow ← Terminal::UserResize ← ControlCore::_refreshSizeUnderLock ← TermControl::_SwapChainSizeChanged. Render thread blocked on the same console lock.
• ASCII control (echo hello) stopped at minimum width and stayed responsive.
• After this change, the same wide-glyph shrink should stop at ~2 columns and stay alive. Needs a rebuilt Dev package to confirm on this machine.

PR Checklist

• Closes Terminal hangs when resizing pane to minimum width with Alt+Shift+Left #19996
• Tests added/passed
  • ScreenSizeLimitsTest now expects the 2-cell floor. DeserializationTests clamps initialRows: -1000000 to 2.
• Documentation updated
• Schema updated (if necessary)

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.

[Leonard Hecker (lhecker)]

This misses GetProposedDimensions and its std::max. I'm not sure if there are more places.

[zhangxaochen]

Leonard Hecker (@lhecker) Thanks — I missed those.

GetProposedDimensions now floors at MINIMUM_VISIBLE_CELLS (2), same as MinimumSize. I also audited the other cell-count floors:

• GetNewDimensions (VT / window-size requests)
• ControlCore create + _refreshSizeUnderLock (the UserResize hang path)
• Terminal::Create / CreateFromSettings / UserResize
• HwndTerminal
• settings initialCols / initialRows clamp + the Launch page minimum

Left alone on purpose:

• TextBuffer / conhost 1×1 — still the remaining Reflow root-cause
• ConptyConnection / AppHost max(..., 1) — PTY size and pixel clamps, not cell counts