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]

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[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

[Leonard Hecker (lhecker)]

/azp run

[azure-pipelines]

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[Leonard Hecker (lhecker)]

/azp run

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[Leonard Hecker (lhecker)]

......wat?

Anyway, you (we?) must merge main into this branch.

[zhangxaochen]

Merged main into this branch to clear the conflict. Launch.xaml kept the new SettingsExpander layout and the 2-cell minimum.

[Leonard Hecker (lhecker)]

/azp run

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[zhangxaochen]

Leonard Hecker (@lhecker) Dustin L. Howett (@DHowett) CI is green and this has an approval. Ready to merge whenever you have a moment.

[Dustin L. Howett (DHowett)]

(Leonard Hecker (@lhecker) why would we merge this instead of just fixing the root cause?)

[Leonard Hecker (lhecker)]

If we fix the root cause, the wide glyphs get dropped entirely.
Any ideal fix would fix the root cause and also this.

[Dustin L. Howett (DHowett)]

I will not merge this pull request with the "cursor agent" attribution in the commit history. Please rebase to remove cursor from your history and indicate to us that you take full ownership of the code herein authored.

[Dustin L. Howett (DHowett)]

as noted

[zhangxaochen]

Removed the Cursor trailer and rebased onto latest main. I take full ownership of the changes in this PR.

On the root-cause question: this PR is the first cut from #19996 (stop 1-cell resize). TextBuffer::Reflow is still separate.

[Dustin L. Howett (DHowett)]

/azp run

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