fix: keep both panes visible in the studio split view

Examples/Markdown4DStudioFMX/Markdown4DStudioFMX.Main.pas

@@ -197,6 +197,12 @@   TMarkdown4DStudioFMXForm = class(TForm, IPadEditorView, IPadShell)
     procedure HandleDocumentHandedOver(const FileName: string);
     procedure SetViewMode(const Mode: TPadViewMode);
     procedure ApplyViewMode;
+    function AvailableSplitWidth: Single;
+    procedure ApplySplitEditorWidth(const DesiredWidth: Single);
+    procedure HandleSplitterMouseUp(Sender: TObject; Button: TMouseButton;
+      Shift: TShiftState; X, Y: Single);
+    procedure HandleTocSplitterMouseUp(Sender: TObject; Button: TMouseButton;
+      Shift: TShiftState; X, Y: Single);
     procedure ShowFindBar;
     procedure ShowReplaceBar;
     procedure CloseFindBar;
@@ -313,6 +319,7 @@ implementation
   Markdown4D.Ast.Interfaces,
   Markdown4DStudio.Text,
   Markdown4DStudio.Outline,
+  Markdown4DStudio.SplitLayout,
   Markdown4DStudio.SessionSync,
   Markdown4DStudio.Workspace,
   Markdown4DStudio.LinkPolicy,
@@ -742,6 +749,7 @@ procedure TMarkdown4DStudioFMXForm.BuildTocPanel;
   FTocSplitter.Parent := Self;
   FTocSplitter.Align := TAlignLayout.Left;
   FTocSplitter.Width := SplitterWidth;
+  FTocSplitter.OnMouseUp := HandleTocSplitterMouseUp;
 end;
 
 procedure TMarkdown4DStudioFMXForm.BuildEditorAndPreview;
@@ -757,6 +765,8 @@ procedure TMarkdown4DStudioFMXForm.BuildEditorAndPreview;
   FMainSplitter.Parent := Self;
   FMainSplitter.Align := TAlignLayout.Left;
   FMainSplitter.Width := SplitterWidth;
+  // FMX splitters have no moved event, so a finished drag is caught here.
+  FMainSplitter.OnMouseUp := HandleSplitterMouseUp;
 
   FPreview := TMarkdownViewer.Create(Self);
   FPreview.Parent := Self;
@@ -1166,7 +1176,11 @@ procedure TMarkdown4DStudioFMXForm.Resize;
   LayoutTitleBar;
 
   if FZenActive then
-    UpdateZenPadding;
+    UpdateZenPadding
+  else if FViewMode = TPadViewMode.Split then
+    // A narrower window must take room from the editor rather than from the
+    // preview, which would otherwise be squeezed to nothing.
+    ApplySplitEditorWidth(FEditor.Width);
 
   if (FPalette <> nil) and FPalette.Visible then
     FPalette.Position.X := (ClientWidth - FPalette.Width) / 2;
@@ -1185,14 +1199,70 @@ procedure TMarkdown4DStudioFMXForm.SetViewMode(const Mode: TPadViewMode);
   ApplyViewMode;
 end;
 
+function TMarkdown4DStudioFMXForm.AvailableSplitWidth: Single;
+begin
+  // What the editor and preview actually have to share.
+  Result := ClientWidth - FMainSplitter.Width;
+
+  if FTocPanel.Visible then
+    Result := Result - FTocPanel.Width;
+  if FTocSplitter.Visible then
+    Result := Result - FTocSplitter.Width;
+end;
+
+procedure TMarkdown4DStudioFMXForm.ApplySplitEditorWidth(const DesiredWidth: Single);
+begin
+  // Entering split view asks for the remembered width; a resize or a contents
+  // pane toggle asks for the width the editor already has, so that a splitter
+  // drag is kept and only trimmed when it no longer fits.
+
+  // The contents pane gives way first, so the two halves keep their minimum for
+  // as long as the window allows.
+  if FTocPanel.Visible then
+    FTocPanel.Width := TPadSplitLayout.ClampSidePanelWidth(Round(FTocPanel.Width),
+      Round(ClientWidth), Round(FTocSplitter.Width + FMainSplitter.Width),
+      MinPaneWidth, MinTocPanelWidth);
+
+  const Available = Round(AvailableSplitWidth);
+
+  // Re-stated on every width change, because a splitter told to honour a
+  // minimum the window cannot give lets a drag collapse the other pane.
+  FMainSplitter.MinSize := TPadSplitLayout.EffectiveMinPaneWidth(Available, MinPaneWidth);
+
+  FEditor.Width := TPadSplitLayout.ClampEditorWidth(Round(DesiredWidth), Available,
+    MinPaneWidth);
+end;
+
+procedure TMarkdown4DStudioFMXForm.HandleSplitterMouseUp(Sender: TObject;
+  Button: TMouseButton; Shift: TShiftState; X, Y: Single);
+begin
+  // A drag is the user stating a preference, so remember it. It still has to
+  // pass the clamp, because on a narrow window the splitter cannot enforce a
+  // minimum the window is too small to give.
+  ApplySplitEditorWidth(FEditor.Width);
+  FSplitEditorWidth := FEditor.Width;
+end;
+
+procedure TMarkdown4DStudioFMXForm.HandleTocSplitterMouseUp(Sender: TObject;
+  Button: TMouseButton; Shift: TShiftState; X, Y: Single);
+begin
+  // Widening the contents pane takes room from the two halves, so it stops
+  // where they would lose their minimum. Only split view reserves room for
+  // two panes; the single pane modes have nothing to protect.
+  if FZenActive or (FViewMode <> TPadViewMode.Split) then
+    Exit;
+
+  ApplySplitEditorWidth(FEditor.Width);
+end;
+
 procedure TMarkdown4DStudioFMXForm.ApplyViewMode;
 begin
   case FViewMode of
     TPadViewMode.Split:
       begin
         FEditor.Visible := True;
         FEditor.Align := TAlignLayout.Left;
-        FEditor.Width := FSplitEditorWidth;
+        ApplySplitEditorWidth(FSplitEditorWidth);
         FMainSplitter.Visible := True;
         FPreview.Visible := True;
       end;
@@ -1639,6 +1709,10 @@ procedure TMarkdown4DStudioFMXForm.ToggleTocPane;
   const ShowToc = not FTocPanel.Visible;
   FTocPanel.Visible := ShowToc;
   FTocSplitter.Visible := ShowToc;
+
+  // Showing the contents pane takes the same room the two halves share.
+  if (not FZenActive) and (FViewMode = TPadViewMode.Split) then
+    ApplySplitEditorWidth(FEditor.Width);
 end;
 
 procedure TMarkdown4DStudioFMXForm.HandleFindClick(Sender: TObject);

Examples/Markdown4DStudioFMX/Markdown4DStudioFMX.dpr

@@ -21,6 +21,7 @@ uses
   Markdown4DStudio.Defines in '..\Shared\Markdown4DStudio.Defines.pas',
   Markdown4DStudio.Text in '..\Shared\Markdown4DStudio.Text.pas',
   Markdown4DStudio.Outline in '..\Shared\Markdown4DStudio.Outline.pas',
+  Markdown4DStudio.SplitLayout in '..\Shared\Markdown4DStudio.SplitLayout.pas',
   Markdown4DStudioFMX.Defines in 'Markdown4DStudioFMX.Defines.pas',
   Markdown4DStudio.Shell in '..\Shared\Markdown4DStudio.Shell.pas',
   Markdown4DStudio.CommandLine in '..\Shared\Markdown4DStudio.CommandLine.pas',

Examples/Markdown4DStudioVCL/Markdown4DStudioVCL.Main.pas

@@ -245,6 +245,10 @@       TPadChrome = record
     function BuildCommandActions: TPadCommandActions;
     procedure SetViewMode(const Mode: TPadViewMode);
     procedure ApplyViewMode;
+    function AvailableSplitWidth: Integer;
+    procedure ApplySplitEditorWidth(const DesiredWidth: Integer);
+    procedure HandleSplitterMoved(Sender: TObject);
+    procedure HandleTocSplitterMoved(Sender: TObject);
     procedure EnforceTopBarOrder;
     procedure EnforceLeftPaneOrder;
     procedure ShowFindBar;
@@ -298,6 +302,7 @@ implementation
   Markdown4DStudio.SessionSync,
   Markdown4DStudio.Text,
   Markdown4DStudio.Outline,
+  Markdown4DStudio.SplitLayout,
   Markdown4DStudio.Workspace,
   Markdown4DStudio.LinkPolicy,
   Markdown4DStudio.SingleInstance,
@@ -368,6 +373,9 @@ procedure TMarkdown4DStudioVCLForm.ConfigureControls;
 begin
   BuildReplaceControls;
   lblFindCount.Caption := EmptyFindCaption;
+
+  splMain.OnMoved := HandleSplitterMoved;
+  splToc.OnMoved := HandleTocSplitterMoved;
 end;
 
 procedure TMarkdown4DStudioVCLForm.BuildToolbar;
@@ -893,6 +901,10 @@ procedure TMarkdown4DStudioVCLForm.ToggleTocPane;
   pnlToc.Visible := ShowToc;
   splToc.Visible := ShowToc;
 
+  // Showing the contents pane takes the same room the two halves share.
+  if (not FZenActive) and (FViewMode = TPadViewMode.Split) then
+    ApplySplitEditorWidth(mdEditor.Width);
+
   EnforceLeftPaneOrder;
 end;
 
@@ -1716,6 +1728,59 @@ procedure TMarkdown4DStudioVCLForm.SetViewMode(const Mode: TPadViewMode);
   ApplyViewMode;
 end;
 
+function TMarkdown4DStudioVCLForm.AvailableSplitWidth: Integer;
+begin
+  // What the editor and preview actually have to share.
+  Result := ClientWidth - splMain.Width;
+
+  if pnlToc.Visible then
+    Dec(Result, pnlToc.Width);
+  if splToc.Visible then
+    Dec(Result, splToc.Width);
+end;
+
+procedure TMarkdown4DStudioVCLForm.ApplySplitEditorWidth(const DesiredWidth: Integer);
+begin
+  // Entering split view asks for the remembered width; a resize or a contents
+  // pane toggle asks for the width the editor already has, so that a splitter
+  // drag is kept and only trimmed when it no longer fits.
+
+  // The contents pane gives way first, so the two halves keep their minimum for
+  // as long as the window allows.
+  if pnlToc.Visible then
+    pnlToc.Width := TPadSplitLayout.ClampSidePanelWidth(pnlToc.Width, ClientWidth,
+      splToc.Width + splMain.Width, MinPaneWidth, MinTocPanelWidth);
+
+  const Available = AvailableSplitWidth;
+
+  // Re-stated on every width change, because a splitter told to honour a
+  // minimum the window cannot give lets a drag collapse the other pane.
+  splMain.MinSize := TPadSplitLayout.EffectiveMinPaneWidth(Available, MinPaneWidth);
+
+  mdEditor.Width := TPadSplitLayout.ClampEditorWidth(DesiredWidth, Available,
+    MinPaneWidth);
+end;
+
+procedure TMarkdown4DStudioVCLForm.HandleSplitterMoved(Sender: TObject);
+begin
+  // A drag is the user stating a preference, so remember it. It still has to
+  // pass the clamp, because on a narrow window the splitter cannot enforce a
+  // minimum the window is too small to give.
+  ApplySplitEditorWidth(mdEditor.Width);
+  FSplitEditorWidth := mdEditor.Width;
+end;
+
+procedure TMarkdown4DStudioVCLForm.HandleTocSplitterMoved(Sender: TObject);
+begin
+  // Widening the contents pane takes room from the two halves, so it stops
+  // where they would lose their minimum. Only split view reserves room for
+  // two panes; the single pane modes have nothing to protect.
+  if FZenActive or (FViewMode <> TPadViewMode.Split) then
+    Exit;
+
+  ApplySplitEditorWidth(mdEditor.Width);
+end;
+
 procedure TMarkdown4DStudioVCLForm.ApplyViewMode;
 begin
   case FViewMode of
@@ -1736,7 +1801,7 @@ procedure TMarkdown4DStudioVCLForm.ApplyViewMode;
     begin
       mdEditor.Align := alLeft;
       mdEditor.Visible := True;
-      mdEditor.Width := FSplitEditorWidth;
+      ApplySplitEditorWidth(FSplitEditorWidth);
       splMain.Visible := True;
       mdPreview.Visible := True;
     end;
@@ -2058,7 +2123,11 @@ procedure TMarkdown4DStudioVCLForm.HandleResize(Sender: TObject);
   LayoutTitleBar;
 
   if FZenActive then
-    UpdateZenPadding;
+    UpdateZenPadding
+  else if FViewMode = TPadViewMode.Split then
+    // A narrower window must take room from the editor rather than from the
+    // preview, which would otherwise be squeezed to nothing.
+    ApplySplitEditorWidth(mdEditor.Width);
 
   if FPalette <> nil then
     FPalette.Left := (ClientWidth - FPalette.Width) div 2;

Examples/Markdown4DStudioVCL/Markdown4DStudioVCL.dpr

@@ -19,6 +19,7 @@ uses
   Markdown4DStudio.Defines in '..\Shared\Markdown4DStudio.Defines.pas',
   Markdown4DStudio.Text in '..\Shared\Markdown4DStudio.Text.pas',
   Markdown4DStudio.Outline in '..\Shared\Markdown4DStudio.Outline.pas',
+  Markdown4DStudio.SplitLayout in '..\Shared\Markdown4DStudio.SplitLayout.pas',
   Markdown4DStudioVCL.Defines in 'Markdown4DStudioVCL.Defines.pas',
   Markdown4DStudio.Shell in '..\Shared\Markdown4DStudio.Shell.pas',
   Markdown4DStudio.CommandLine in '..\Shared\Markdown4DStudio.CommandLine.pas',

Examples/Shared/Markdown4DStudio.Defines.pas

@@ -55,6 +55,12 @@ interface
   SeparatorWidth = 1;
   FindEditWidth = 160;
   TocPanelWidth = 240;
+  // Neither half of the split view may be squeezed below this, whether by a
+  // remembered width, a window resize or a splitter drag.
+  MinPaneWidth = 300;
+  // The contents pane gives way first to protect those two, but never below
+  // this; past that point the two halves share what is left between them.
+  MinTocPanelWidth = 120;
   TickIntervalMilliseconds = 100;
   FindBarHeight = 32;
   FindBarEditWidth = 240;

Examples/Shared/Markdown4DStudio.SplitLayout.pas

@@ -0,0 +1,88 @@
+unit Markdown4DStudio.SplitLayout;
+
+// Geometry for the editor/preview split, deliberately free of any framework type
+// so the rule that neither pane may collapse is testable without a form and is
+// shared by the VCL and FMX studios.
+
+interface
+
+type
+  TPadSplitLayout = record
+  public
+    /// <summary>
+    ///   Editor width to apply for the window as it is now.
+    /// </summary>
+    /// <param name="SavedWidth">
+    ///   Editor width remembered from the last time split view was active. It is
+    ///   an absolute pixel value, so it can outlive the window size it was taken
+    ///   at and ask for more room than there is.
+    /// </param>
+    /// <param name="AvailableWidth">
+    ///   What the editor and the preview share, with the contents pane and every
+    ///   splitter already subtracted.
+    /// </param>
+    /// <param name="MinPaneWidth">
+    ///   The least either pane may become. When the available width cannot hold
+    ///   two of these, the space is shared evenly instead.
+    /// </param>
+    class function ClampEditorWidth(const SavedWidth, AvailableWidth,
+      MinPaneWidth: Integer): Integer; static;
+
+    /// <summary>
+    ///   The minimum a splitter can actually enforce for the window as it is
+    ///   now. A splitter asked to honour a minimum the window cannot give lets a
+    ///   drag hand the whole area to one pane, so below two minimums the answer
+    ///   is half of what there is.
+    /// </summary>
+    class function EffectiveMinPaneWidth(const AvailableWidth,
+      MinPaneWidth: Integer): Integer; static;
+
+    /// <summary>
+    ///   How wide a side panel, such as the contents pane, may be dragged before
+    ///   it starts eating into the two halves of the split.
+    /// </summary>
+    /// <param name="TotalWidth">The whole client width.</param>
+    /// <param name="ReservedWidth">Width taken by the splitters themselves.</param>
+    /// <param name="MinSidePanelWidth">
+    ///   How far the side panel may be asked to give way. Past that point it has
+    ///   done all it can and the two panes share what is left between them. A
+    ///   panel already narrower than this is never widened.
+    /// </param>
+    class function ClampSidePanelWidth(const DesiredWidth, TotalWidth,
+      ReservedWidth, MinPaneWidth, MinSidePanelWidth: Integer): Integer; static;
+  end;
+
+implementation
+
+uses
+  System.Math;
+
+class function TPadSplitLayout.ClampEditorWidth(const SavedWidth, AvailableWidth,
+  MinPaneWidth: Integer): Integer;
+begin
+  // Too narrow to honour both minimums: share what there is rather than hand the
+  // whole area to one pane and leave the other invisible.
+  if AvailableWidth <= 2 * MinPaneWidth then
+    Exit(Max(0, AvailableWidth div 2));
+
+  Result := EnsureRange(SavedWidth, MinPaneWidth, AvailableWidth - MinPaneWidth);
+end;
+
+class function TPadSplitLayout.EffectiveMinPaneWidth(const AvailableWidth,
+  MinPaneWidth: Integer): Integer;
+begin
+  Result := Min(MinPaneWidth, Max(0, AvailableWidth div 2));
+end;
+
+class function TPadSplitLayout.ClampSidePanelWidth(const DesiredWidth, TotalWidth,
+  ReservedWidth, MinPaneWidth, MinSidePanelWidth: Integer): Integer;
+begin
+  // What the side panel could take while both panes still keep their minimum.
+  const Allowed = Max(0, TotalWidth - ReservedWidth - 2 * MinPaneWidth);
+
+  // Never widen the panel, only ask it to give way, and never past its own
+  // minimum: below that the two panes share what is left instead.
+  Result := Min(DesiredWidth, Max(Allowed, MinSidePanelWidth));
+end;
+
+end.

Tests/Markdown4D.Tests.dpr

@@ -89,6 +89,7 @@ uses
   Markdown4DStudio.Defines in '..\Examples\Shared\Markdown4DStudio.Defines.pas',
   Markdown4DStudio.Text in '..\Examples\Shared\Markdown4DStudio.Text.pas',
   Markdown4DStudio.Outline in '..\Examples\Shared\Markdown4DStudio.Outline.pas',
+  Markdown4DStudio.SplitLayout in '..\Examples\Shared\Markdown4DStudio.SplitLayout.pas',
   Markdown4DStudio.CommandLine in '..\Examples\Shared\Markdown4DStudio.CommandLine.pas',
   Markdown4DStudio.Shell in '..\Examples\Shared\Markdown4DStudio.Shell.pas',
   Markdown4DStudio.EditorView in '..\Examples\Shared\Markdown4DStudio.EditorView.pas',
@@ -105,6 +106,7 @@ uses
   Markdown4DStudio.Controller.Tests in 'Markdown4DStudio.Controller.Tests.pas',
   Markdown4DStudio.HtmlExport.Tests in 'Markdown4DStudio.HtmlExport.Tests.pas',
   Markdown4DStudio.LinkPolicy.Tests in 'Markdown4DStudio.LinkPolicy.Tests.pas',
+  Markdown4DStudio.SplitLayout.Tests in 'Markdown4DStudio.SplitLayout.Tests.pas',
   Markdown4D.Text.UrlSafety.Tests in 'Markdown4D.Text.UrlSafety.Tests.pas',
   Markdown4D.Viewer.ImageSettings.Tests in 'Markdown4D.Viewer.ImageSettings.Tests.pas',
   StreamingMarkdown.Demo.Tests in 'StreamingMarkdown.Demo.Tests.pas',