System.ArgumentOutOfRangeException when undoing text on a semi-transparent layer and when attempting subsequent edits, crash

[Sanjowolf]

Description
I was editing a two-layer project, and added some text on a the top semi-transparent layer; I then tried to undo this text, and suddenly the two layers merged ("Layer 3" was the bottom layer, see screenshot) and the canvas went blank, as I got this error pop-up:

System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')
   at Pinta.Tools.TextTool.RedrawText(Boolean showCursor, Boolean useTextLayer)
   at Pinta.Tools.TextTool.UpdateMouseCursor(Document document)
   at Pinta.Tools.TextTool.OnMouseMove(Document document, ToolMouseEventArgs e)
   at Pinta.CanvasWindow.HandleMotion(EventControllerMotion controller, MotionSignalArgs args)
   at GObject.Closure.InternalCallback(IntPtr closure, IntPtr returnValuePtr, UInt32 nParamValues, IntPtr paramValuesData, IntPtr invocationHint, IntPtr userData)

Attempting to do almost anything, including zooming or brushing, also yields non-stop pop-up errors. I could still hide and show the layer without any error, but the canvas remained entirely blank. I was getting this one when I tried to undo the last action:

System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')
   at System.Collections.Generic.List`1.get_Item(Int32 index)
   at Pinta.Core.DocumentLayers.get_Item(Int32 index)
   at Pinta.Core.SwapLayersHistoryItem.Swap()
   at Pinta.Core.SwapLayersHistoryItem.Undo()
   at Pinta.Core.DocumentHistory.Undo()
   at Pinta.Core.EditActions.HandlerPintaCoreActionsEditUndoActivated(Object sender, EventArgs e)
   at Pinta.Core.Command.<.ctor>b__33_0(SimpleAction o, ActivateSignalArgs args)
   at GObject.Closure.InternalCallback(IntPtr closure, IntPtr returnValuePtr, UInt32 nParamValues, IntPtr paramValuesData, IntPtr invocationHint, IntPtr userData)

When I then tried selecting an area with the rectangular selection tool, Pinta crashed upon releasing the mouse.

To Reproduce
Unable to reproduce with the same steps in the same file nor in a minimal similar project.

Additional Info

Version
Pinta 3.1.2 Flatpak on openSUSE Tumbleweed.

[cameronwhite]

I was able to reproduce a similar error as the second stack trace you posted with these steps:

• Add a layer
• Use the text tool to type some text
• Use the Move Layer Down button to swap the layer order
• Undo

Pinta: Unhandled exception
System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')
   at System.Collections.Generic.List`1.get_Item(Int32 index)
   at Pinta.Core.DocumentLayers.get_CurrentUserLayer() in /Users/cameron/code/Pinta/Pinta.Core/Classes/DocumentLayers.cs:line 39
   at Pinta.Tools.TextTool.get_CurrentTextEngine() in /Users/cameron/code/Pinta/Pinta.Tools/Tools/TextTool.cs:line 58
   at Pinta.Tools.TextTool.UpdateFont() in /Users/cameron/code/Pinta/Pinta.Tools/Tools/TextTool.cs:line 643
   at Pinta.Tools.TextTool.HandleSelectedLayerChanged(Object sender, EventArgs e) in /Users/cameron/code/Pinta/Pinta.Tools/Tools/TextTool.cs:line 626
   at Pinta.Core.WorkspaceManager.Document_LayerRemoved(Object sender, IndexEventArgs e) in /Users/cameron/code/Pinta/Pinta.Core/Managers/WorkspaceManager.cs:line 241
   at Pinta.Core.DocumentLayers.DeleteLayer(Int32 index) in /Users/cameron/code/Pinta/Pinta.Core/Classes/DocumentLayers.cs:line 183
   at Pinta.Core.SwapLayersHistoryItem.Swap() in /Users/cameron/code/Pinta/Pinta.Core/HistoryItems/SwapLayersHistoryItem.cs:line 67
   at Pinta.Core.SwapLayersHistoryItem.Undo() in /Users/cameron/code/Pinta/Pinta.Core/HistoryItems/SwapLayersHistoryItem.cs:line 46
   at Pinta.Core.DocumentHistory.Undo() in /Users/cameron/code/Pinta/Pinta.Core/Classes/DocumentHistory.cs:line 98
   at Pinta.Core.EditActions.HandlerPintaCoreActionsEditUndoActivated(Object sender, EventArgs e) in /Users/cameron/code/Pinta/Pinta.Core/Actions/EditActions.cs:line 459
   at Pinta.Core.Command.<.ctor>b__33_0(SimpleAction o, ActivateSignalArgs args) in /Users/cameron/code/Pinta/Pinta.Core/Actions/Command.cs:line 72
   at GObject.Closure.InternalCallback(IntPtr closure, IntPtr returnValuePtr, UInt32 nParamValues, IntPtr paramValuesData, IntPtr invocationHint, IntPtr userData)

In this specific case, I think what happens is that the SwapLayersHistoryItem deletes the two layers and then inserts them in the new locations. If there are only two layers then there's a temporary state where the document has no layers, which is problematic for code that doesn't handle that properly when the intermediate layer deletion events are fired

I'm not sure if this approach is really necessary - it might be better to implement the swapping in DocumentLayers and only fire a more minimal set of events? e.g. maybe only a SelectedLayerChanged event is necessary