Add awareness for cursor position in selection-related behaviors across desktop platforms

My recollection from working on Gecko macOS text selection many years ago is that desktop platforms have different expected behaviors around how keyboard text selection interacts with existing selections, especially mouse-based. In particular: if you select some text, then shift-left-arrow or shift-right-arrow, what happens? Does it matter if the text was selected from left to right or right to left with the mouse?

We should audit this behavior on each desktop platform in a native text field, then make sure the framework handling matches on each platform.

@robert-ancell

Related note: @robert-ancell noticed that the base and extent reported by the framework doesn't seem to be different for left-to-right vs right-to-left selection, which may be an issue for getting correct behavior everywhere.

Tl;dr: All native platforms agree on this behavior, but Flutter behaves differently and needs to be updated.

The pivot point is the edge of the selection that remains fixed during the shift/arrow operations before any reversal. Here is an example of each on Mac:

Pivot point left	Pivot point right

For a selection in the middle of the text made with a mouse in the given direction, the behavior is given when holding shift and using the arrow keys.

	Flutter	OSX 10.15.7 TextEdit	Windows 10 Home Notepad	Ubuntu 20.04.1 LTS TextEditor
Left to right	Pivot point right	Pivot point right	Pivot point right	Pivot point right
Right to left	Pivot point right	Pivot point left	Pivot point left	Pivot point left

Is there anything else I'm missing that I should try? Otherwise it seems that Flutter just needs to adjust its behavior to match all the other platforms (no need to adjust per-platform).

I confirmed that Flutter's TextSelection has its base set to the lowest point and extent set to the highest, regardless of the direction of the selection. So indeed, TextSelection does not currently have the information we need to change the pivot point unfortunately.

Here is where we are swapping base and extent to be in ascending order in the codebase:

flutter/packages/flutter/lib/src/rendering/editable.dart

Lines 1854 to 1857 in b372c52

    
           if (toPosition != null) { 
        
             baseOffset = math.min(fromPosition.offset, toPosition.offset); 
        
             extentOffset = math.max(fromPosition.offset, toPosition.offset); 
        
           }

Edit This is now fixed in #71756.

Sorry, this shouldn't have been closed since #71756 only fixes the first problem mentioned (#57679 (comment)) and not the other problems I found in #57679 (comment).

@csells

/cc @csells, who did some of this in Q3.

@justinmc

@justinmc We think this may be done --- can you confirm and close if corrrect? Thanks!

PR #71756 fixed the main problem, but the issue kind of expanded in scope when I went through all of the inconsistencies on all the different platforms in #57679 (comment).

I say let's keep this open until I can match all of the behavior in the table in that comment. Or I can always move that to a separate issue if you think that's better.

@justinmc

@justinmc if we're going to keep this one open rather than open a new bug, it would be useful to update the above table to show which behaviours are working and which still need to be done.

Good call. I've updated the table in #57679 (comment) to match the current behavior. #75032 will fix some of these and I will update the table again when it's merged.

I've updated the table of behaviors at #57679 (comment) now that #75032 has been merged. It looks like it actually fixed a few of them, but there are still some differences that need to be manually tweaked.

I've created this issue to track Windows inconsistencies: #78572.

This PR will fix all Mac problems here and will close this issue: #78573.

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.

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Add awareness for cursor position in selection-related behaviors across desktop platforms #57679

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	if (toPosition != null) {
	baseOffset = math.min(fromPosition.offset, toPosition.offset);
	extentOffset = math.max(fromPosition.offset, toPosition.offset);
	}

Add awareness for cursor position in selection-related behaviors across desktop platforms #57679

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stuartmorgan-g commented on May 20, 2020

justinmc commented on Dec 5, 2020

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justinmc commented on Dec 14, 2020

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justinmc commented on Mar 3, 2021

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justinmc commented on Mar 10, 2021

justinmc commented on Mar 18, 2021

justinmc commented on Mar 18, 2021

github-actions commented on Aug 4, 2021

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