Prevent panes from shrinking below 2x2 cells (#20564)

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 mitigates an issue where we hang when we display a 2-cell char
in one cell.

Refs:
- j4james: 2×2 is the VT theoretical minimum (`DECSTBM` height ≥ 2,
`DECSLRM` width ≥ 2).

Verification:
- 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.

Closes #19996

(cherry picked from commit f88a1cdafd)

Co-authored-by: Leonard Hecker <lhecker@microsoft.com>
Service-Version: 1.24

src/cascadia/TerminalApp/Pane.cpp

@@ -2844,13 +2844,12 @@ void Pane::_AdvanceSnappedDimension(const bool widthOrHeight, LayoutSizeNode& si
 
 // Method Description:
 // - Get the absolute minimum size that this pane can be resized to and still
-//   have 1x1 character visible, in each of its children. If we're a leaf, we'll
-//   include the space needed for borders _within_ us.
+//   satisfy each child's MinimumSize. If we're a leaf, we'll include the
+//   space needed for borders _within_ us.
 // Arguments:
 // - <none>
 // Return Value:
-// - The minimum size that this pane can be resized to and still have a visible
-//   character.
+// - The minimum size that this pane can be resized to and still fit its content.
 Size Pane::_GetMinSize() const
 {
     if (_IsLeaf())

src/cascadia/TerminalControl/ControlCore.cpp

@@ -405,12 +405,12 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             LOG_IF_FAILED(_renderEngine->SetWindowSize({ viewInPixels.Width(), viewInPixels.Height() }));
 
             const auto vp = _renderEngine->GetViewportInCharacters(viewInPixels);
-            const auto width = vp.Width();
-            const auto height = vp.Height();
+            const til::size viewportSize{ Utils::ClampToShortMax(vp.Width(), MINIMUM_VISIBLE_CELLS),
+                                          Utils::ClampToShortMax(vp.Height(), MINIMUM_VISIBLE_CELLS) };
 
             if (_connection)
             {
-                _connection.Resize(height, width);
+                _connection.Resize(viewportSize.height, viewportSize.width);
             }
 
             if (_owningHwnd != 0)
@@ -422,8 +422,8 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             }
 
             // Override the default width and height to match the size of the swapChainPanel
-            _settings->InitialCols(width);
-            _settings->InitialRows(height);
+            _settings->InitialCols(viewportSize.width);
+            _settings->InitialRows(viewportSize.height);
 
             _terminal->CreateFromSettings(*_settings, *_renderer);
 
@@ -1167,10 +1167,12 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         auto cx = gsl::narrow_cast<til::CoordType>(lrint(_panelWidth * _compositionScale));
         auto cy = gsl::narrow_cast<til::CoordType>(lrint(_panelHeight * _compositionScale));
 
-        // Don't actually resize so small that a single character wouldn't fit
-        // in either dimension. The buffer really doesn't like being size 0.
-        cx = std::max(cx, _actualFont.GetSize().width);
-        cy = std::max(cy, _actualFont.GetSize().height);
+        // Don't resize below the visible minimum. A 1-cell viewport can hang
+        // TextBuffer::Reflow on a wide glyph (GH#19996). The buffer also
+        // doesn't like being size 0.
+        const auto cell = _actualFont.GetSize();
+        cx = std::max(cx, cell.width * MINIMUM_VISIBLE_CELLS);
+        cy = std::max(cy, cell.height * MINIMUM_VISIBLE_CELLS);
 
         // Convert our new dimensions to characters
         const auto viewInPixels = Viewport::FromDimensions({ 0, 0 }, { cx, cy });
@@ -1186,7 +1188,9 @@ namespace winrt::Microsoft::Terminal::Control::implementation
 
         // If this function succeeds with S_FALSE, then the terminal didn't
         // actually change size. No need to notify the connection of this no-op.
-        const auto hr = _terminal->UserResize({ vp.Width(), vp.Height() });
+        const auto cols = std::max(vp.Width(), MINIMUM_VISIBLE_CELLS);
+        const auto rows = std::max(vp.Height(), MINIMUM_VISIBLE_CELLS);
+        const auto hr = _terminal->UserResize({ cols, rows });
         if (FAILED(hr) || hr == S_FALSE)
         {
             return;
@@ -1194,7 +1198,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
 
         if (_connection)
         {
-            _connection.Resize(vp.Height(), vp.Width());
+            _connection.Resize(rows, cols);
         }
 
         // TermControl will call Search() once the OutputIdle even fires after 100ms.

src/cascadia/TerminalControl/HwndTerminal.cpp

@@ -434,10 +434,10 @@ HRESULT HwndTerminal::Refresh(const til::size windowSize, _Out_ til::size* dimen
     const auto viewInPixels = Viewport::FromDimensions({}, windowSize);
     const auto vp = _renderEngine->GetViewportInCharacters(viewInPixels);
 
-    // Guard against resizing the window to 0 columns/rows, which the text buffer classes don't really support.
+    // Guard against resizing below the visible minimum (GH#19996).
     auto size = vp.Dimensions();
-    size.width = std::max(size.width, 1);
-    size.height = std::max(size.height, 1);
+    size.width = std::max(size.width, MINIMUM_VISIBLE_CELLS);
+    size.height = std::max(size.height, MINIMUM_VISIBLE_CELLS);
 
     // If this function succeeds with S_FALSE, then the terminal didn't
     //      actually change size. No need to notify the connection of this
@@ -575,8 +575,8 @@ try
     const auto lock = publicTerminal->_terminal->LockForReading();
     const auto viewInCharacters = publicTerminal->_renderEngine->GetViewportInCharacters(viewInPixels);
 
-    dimensions->width = viewInCharacters.Width();
-    dimensions->height = viewInCharacters.Height();
+    dimensions->width = std::max(viewInCharacters.Width(), MINIMUM_VISIBLE_CELLS);
+    dimensions->height = std::max(viewInCharacters.Height(), MINIMUM_VISIBLE_CELLS);
 
     return S_OK;
 }

src/cascadia/TerminalControl/TermControl.cpp

@@ -5,6 +5,7 @@
 #include "TermControl.h"
 
 #include <LibraryResources.h>
+#include <DefaultSettings.h>
 #include <inputpaneinterop.h>
 
 #include "TermControlAutomationPeer.h"
@@ -2840,16 +2841,18 @@ namespace winrt::Microsoft::Terminal::Control::implementation
                                                                         int32_t commandlineRows)
     {
         // If the settings have negative or zero row or column counts, ignore those counts.
+        // Floor at MINIMUM_VISIBLE_CELLS so wt --size 1,1 / initialCols:1 cannot
+        // open a 1-cell viewport (GH#19996).
         // (The lower TerminalCore layer also has upper bounds as well, but at this layer
         //  we may eventually impose different ones depending on how many pixels we can address.)
         const auto cols = static_cast<float>(std::max(commandlineCols > 0 ?
                                                           commandlineCols :
                                                           settings.InitialCols(),
-                                                      1));
+                                                      MINIMUM_VISIBLE_CELLS));
         const auto rows = static_cast<float>(std::max(commandlineRows > 0 ?
                                                           commandlineRows :
                                                           settings.InitialRows(),
-                                                      1));
+                                                      MINIMUM_VISIBLE_CELLS));
 
         const winrt::Windows::Foundation::Size initialSize{ cols, rows };
 
@@ -2941,8 +2944,8 @@ namespace winrt::Microsoft::Terminal::Control::implementation
     // - a size containing the requested dimensions in pixels.
     winrt::Windows::Foundation::Size TermControl::GetNewDimensions(const winrt::Windows::Foundation::Size& sizeInChars)
     {
-        const auto cols = ::base::saturated_cast<int32_t>(sizeInChars.Width);
-        const auto rows = ::base::saturated_cast<int32_t>(sizeInChars.Height);
+        const auto cols = std::max(::base::saturated_cast<int32_t>(sizeInChars.Width), MINIMUM_VISIBLE_CELLS);
+        const auto rows = std::max(::base::saturated_cast<int32_t>(sizeInChars.Height), MINIMUM_VISIBLE_CELLS);
         const auto fontSize = _core.FontSize();
         const auto scrollState = _core.Settings().ScrollState();
         const auto padding = _core.Settings().Padding();
@@ -2983,20 +2986,22 @@ namespace winrt::Microsoft::Terminal::Control::implementation
 
     // Method Description:
     // - Get the absolute minimum size that this control can be resized to and
-    //   still have 1x1 character visible. This includes the space needed for
+    //   still have 2x2 characters visible. This includes the space needed for
     //   the scrollbar and the padding.
+    //   2x2 is the VT theoretical minimum (DECSTBM / DECSLRM). A 1-cell
+    //   viewport can hang TextBuffer::Reflow on a wide glyph (GH#19996).
     // Arguments:
     // - <none>
     // Return Value:
     // - The minimum size that this terminal control can be resized to and still
-    //   have a visible character.
+    //   have a usable character grid.
     winrt::Windows::Foundation::Size TermControl::MinimumSize()
     {
         if (_initializedTerminal)
         {
             const auto fontSize = _core.FontSizeInDips();
-            auto width = fontSize.Width;
-            auto height = fontSize.Height;
+            auto width = fontSize.Width * MINIMUM_VISIBLE_CELLS;
+            auto height = fontSize.Height * MINIMUM_VISIBLE_CELLS;
             // Reserve additional space if scrollbar is intended to be visible
             if (_core.Settings().ScrollState() != ScrollbarState::Hidden)
             {

src/cascadia/TerminalCore/Terminal.cpp

@@ -42,6 +42,8 @@ Terminal::Terminal(TestDummyMarker) :
 
 void Terminal::Create(til::size viewportSize, til::CoordType scrollbackLines, Renderer& renderer)
 {
+    viewportSize.width = std::max(viewportSize.width, MINIMUM_VISIBLE_CELLS);
+    viewportSize.height = std::max(viewportSize.height, MINIMUM_VISIBLE_CELLS);
     _mutableViewport = Viewport::FromDimensions({ 0, 0 }, viewportSize);
     _scrollbackLines = scrollbackLines;
     const til::size bufferSize{ viewportSize.width,
@@ -75,8 +77,8 @@ void Terminal::HardResetWithoutErase()
 void Terminal::CreateFromSettings(ICoreSettings settings,
                                   Renderer& renderer)
 {
-    const til::size viewportSize{ Utils::ClampToShortMax(settings.InitialCols(), 1),
-                                  Utils::ClampToShortMax(settings.InitialRows(), 1) };
+    const til::size viewportSize{ Utils::ClampToShortMax(settings.InitialCols(), MINIMUM_VISIBLE_CELLS),
+                                  Utils::ClampToShortMax(settings.InitialRows(), MINIMUM_VISIBLE_CELLS) };
 
     // TODO:MSFT:20642297 - Support infinite scrollback here, if HistorySize is -1
     Create(viewportSize, Utils::ClampToShortMax(settings.HistorySize(), 0), renderer);
@@ -275,9 +277,14 @@ std::wstring_view Terminal::GetWorkingDirectory() noexcept
 // - S_OK if we successfully resized the terminal, S_FALSE if there was
 //      nothing to do (the viewportSize is the same as our current size), or an
 //      appropriate HRESULT for failing to resize.
-[[nodiscard]] HRESULT Terminal::UserResize(const til::size viewportSize) noexcept
+[[nodiscard]] HRESULT Terminal::UserResize(const til::size requestedSize) noexcept
 try
 {
+    const til::size viewportSize{
+        std::max(requestedSize.width, MINIMUM_VISIBLE_CELLS),
+        std::max(requestedSize.height, MINIMUM_VISIBLE_CELLS)
+    };
+
     const auto oldDimensions = _GetMutableViewport().Dimensions();
     if (viewportSize == oldDimensions)
     {

src/cascadia/TerminalSettingsEditor/Launch.xaml

@@ -28,7 +28,7 @@
                    TargetType="muxc:NumberBox">
                 <Setter Property="SmallChange" Value="1" />
                 <Setter Property="LargeChange" Value="10" />
-                <Setter Property="Minimum" Value="1" />
+                <Setter Property="Minimum" Value="2" />
             </Style>
             <Style x:Key="LaunchPositionNumberBoxStyle"
                    BasedOn="{StaticResource NumberBoxSettingStyle}"
@@ -213,7 +213,7 @@
                                     Grid.Column="1"
                                     VerticalAlignment="Center"
                                     Maximum="999"
-                                    Minimum="1"
+                                    Minimum="2"
                                     Style="{StaticResource LaunchSizeNumberBoxStyle}"
                                     Value="{x:Bind ViewModel.InitialCols, Mode=TwoWay}" />
                     <TextBlock x:Uid="Globals_InitialRows"
@@ -226,7 +226,7 @@
                                     Grid.Column="1"
                                     VerticalAlignment="Center"
                                     Maximum="999"
-                                    Minimum="1"
+                                    Minimum="2"
                                     Style="{StaticResource LaunchSizeNumberBoxStyle}"
                                     Value="{x:Bind ViewModel.InitialRows, Mode=TwoWay}" />
                 </Grid>

src/cascadia/TerminalSettingsModel/GlobalAppSettings.cpp

@@ -3,6 +3,7 @@
 
 #include "pch.h"
 #include "GlobalAppSettings.h"
+#include <DefaultSettings.h>
 #include "../../types/inc/Utils.hpp"
 #include "JsonUtils.h"
 #include "KeyChordSerialization.h"
@@ -182,11 +183,11 @@ void GlobalAppSettings::LayerJson(const Json::Value& json, const OriginTag origi
     // otherwise we could end up setting defaults that get persisted
     if (this->HasInitialCols())
     {
-        this->InitialCols(std::clamp(this->InitialCols(), 1, 999));
+        this->InitialCols(std::clamp(this->InitialCols(), MINIMUM_VISIBLE_CELLS, 999));
     }
     if (this->HasInitialRows())
     {
-        this->InitialRows(std::clamp(this->InitialRows(), 1, 999));
+        this->InitialRows(std::clamp(this->InitialRows(), MINIMUM_VISIBLE_CELLS, 999));
     }
     LayerActionsFrom(json, origin, true);
 

src/cascadia/UnitTests_SettingsModel/DeserializationTests.cpp

@@ -1274,7 +1274,7 @@ namespace SettingsModelUnitTests
         const auto settings = createSettings(inputSettings);
 
         VERIFY_ARE_EQUAL(999, settings->GlobalSettings().InitialCols());
-        VERIFY_ARE_EQUAL(1, settings->GlobalSettings().InitialRows());
+        VERIFY_ARE_EQUAL(MINIMUM_VISIBLE_CELLS, settings->GlobalSettings().InitialRows());
     }
 
     void DeserializationTests::TestTrailingCommas()

src/cascadia/UnitTests_TerminalCore/ScreenSizeLimitsTest.cpp

@@ -36,21 +36,21 @@ using namespace TerminalCoreUnitTests;
 void ScreenSizeLimitsTest::ScreenWidthAndHeightAreClampedToBounds()
 {
     // Negative values for initial visible row count or column count
-    // are clamped to 1. Too-large positive values are clamped to SHRT_MAX.
+    // are clamped to MINIMUM_VISIBLE_CELLS. Too-large positive values are clamped to SHRT_MAX.
     auto negativeColumnsSettings = winrt::make<MockTermSettings>(10000, 9999999, -1234);
     Terminal negativeColumnsTerminal{ Terminal::TestDummyMarker{} };
     DummyRenderer renderer{ &negativeColumnsTerminal };
     negativeColumnsTerminal.CreateFromSettings(negativeColumnsSettings, renderer);
     auto actualDimensions = negativeColumnsTerminal.GetViewport().Dimensions();
     VERIFY_ARE_EQUAL(actualDimensions.height, SHRT_MAX, L"Row count clamped to SHRT_MAX == " WCS(SHRT_MAX));
-    VERIFY_ARE_EQUAL(actualDimensions.width, 1, L"Column count clamped to 1");
+    VERIFY_ARE_EQUAL(actualDimensions.width, MINIMUM_VISIBLE_CELLS, L"Column count clamped to MINIMUM_VISIBLE_CELLS");
 
-    // Zero values are clamped to 1 as well.
+    // Zero values are clamped to MINIMUM_VISIBLE_CELLS as well.
     auto zeroRowsSettings = winrt::make<MockTermSettings>(10000, 0, 9999999);
     Terminal zeroRowsTerminal{ Terminal::TestDummyMarker{} };
     zeroRowsTerminal.CreateFromSettings(zeroRowsSettings, renderer);
     actualDimensions = zeroRowsTerminal.GetViewport().Dimensions();
-    VERIFY_ARE_EQUAL(actualDimensions.height, 1, L"Row count clamped to 1");
+    VERIFY_ARE_EQUAL(actualDimensions.height, MINIMUM_VISIBLE_CELLS, L"Row count clamped to MINIMUM_VISIBLE_CELLS");
     VERIFY_ARE_EQUAL(actualDimensions.width, SHRT_MAX, L"Column count clamped to SHRT_MAX == " WCS(SHRT_MAX));
 }
 

src/cascadia/WpfTerminalControl/TerminalContainer.cs

@@ -225,8 +225,8 @@ namespace Microsoft.Terminal.Wpf
 
             NativeMethods.TilSize dimensions = new NativeMethods.TilSize
             {
-                X = (int)columns,
-                Y = (int)rows,
+                X = Math.Max((int)columns, 2),
+                Y = Math.Max((int)rows, 2),
             };
 
             NativeMethods.TerminalTriggerResizeWithDimension(this.terminal, dimensions, out var dimensionsInPixels);

src/inc/DefaultSettings.h

@@ -33,6 +33,10 @@ constexpr uint16_t DEFAULT_FONT_WEIGHT = 400; // normal
 constexpr int DEFAULT_ROWS = 30;
 constexpr int DEFAULT_COLS = 120;
 
+// VT theoretical minimum (DECSTBM / DECSLRM). A 1-cell viewport can hang
+// TextBuffer::Reflow on a wide glyph (GH#19996).
+constexpr int MINIMUM_VISIBLE_CELLS = 2;
+
 constexpr std::wstring_view DEFAULT_PADDING{ L"8, 8, 8, 8" };
 constexpr std::wstring_view DEFAULT_STARTING_DIRECTORY{ L"%USERPROFILE%" };