Remove hard line breaks when rewording commits

... and when recalling a commit message from an old commit by pressing up-arrow.

This is necessary because committing turns our soft line breaks into real ones,
but when rewording we want to turn them back into soft ones again, so that it's
possible to insert words at the beginning of a paragraph and have everything
rewrap nicely.

This is only a best effort; the algorithm only removes those hard line breaks
that can be removed without changing the way the message looks. This works well
when the previous commit message was wrapped at the same width, which for most
users should be the most common case; but if it wasn't, the result is not great.
Specifically, if the old wrap width was smaller, some hard line breaks just
won't be removed; if it was wider though, you'll get an unpleasant comb effect
with alternating long and short lines. In such a case it's best to switch to the
editor and use whatever wrapping features you have there (e.g. alt-Q).

pkg/gui/controllers/commit_message_controller.go

@@ -3,6 +3,7 @@ package controllers
 import (
 	"github.com/jesseduffield/lazygit/pkg/commands/git_commands"
 	"github.com/jesseduffield/lazygit/pkg/gui/context"
+	"github.com/jesseduffield/lazygit/pkg/gui/controllers/helpers"
 	"github.com/jesseduffield/lazygit/pkg/gui/types"
 )
 
@@ -119,6 +120,9 @@ func (self *CommitMessageController) setCommitMessageAtIndex(index int) (bool, e
 		}
 		return false, self.c.ErrorMsg(self.c.Tr.CommitWithoutMessageErr)
 	}
+	if self.c.UserConfig.Git.Commit.AutoWrapCommitMessage {
+		commitMessage = helpers.TryRemoveHardLineBreaks(commitMessage, self.c.UserConfig.Git.Commit.AutoWrapWidth)
+	}
 	self.c.Helpers().Commits.UpdateCommitPanelView(commitMessage)
 	return true, nil
 }

pkg/gui/controllers/helpers/commits_helper.go

@@ -5,6 +5,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/jesseduffield/gocui"
 	"github.com/jesseduffield/lazygit/pkg/gui/types"
 	"github.com/samber/lo"
 )
@@ -63,6 +64,31 @@ func (self *CommitsHelper) JoinCommitMessageAndUnwrappedDescription() string {
 	return self.getCommitSummary() + "\n" + self.getUnwrappedCommitDescription()
 }
 
+func TryRemoveHardLineBreaks(message string, autoWrapWidth int) string {
+	messageRunes := []rune(message)
+	lastHardLineStart := 0
+	for i, r := range messageRunes {
+		if r == '\n' {
+			// Try to make this a soft linebreak by turning it into a space, and
+			// checking whether it still wraps to the same result then.
+			messageRunes[i] = ' '
+
+			_, cursorMapping := gocui.AutoWrapContent(messageRunes[lastHardLineStart:], autoWrapWidth)
+
+			// Look at the cursorMapping to check whether auto-wrapping inserted
+			// a line break. If it did, there will be a cursorMapping entry with
+			// Orig pointing to the position after the inserted line break.
+			if len(cursorMapping) == 0 || cursorMapping[0].Orig != i-lastHardLineStart+1 {
+				// It didn't, so change it back to a newline
+				messageRunes[i] = '\n'
+			}
+			lastHardLineStart = i + 1
+		}
+	}
+
+	return string(messageRunes)
+}
+
 func (self *CommitsHelper) SwitchToEditor() error {
 	if !self.c.Contexts().CommitMessage.CanSwitchToEditor() {
 		return nil

pkg/gui/controllers/helpers/commits_helper_test.go

@@ -0,0 +1,41 @@
+package helpers
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestTryRemoveHardLineBreaks(t *testing.T) {
+	scenarios := []struct {
+		name           string
+		message        string
+		autoWrapWidth  int
+		expectedResult string
+	}{
+		{
+			name:           "empty",
+			message:        "",
+			autoWrapWidth:  7,
+			expectedResult: "",
+		},
+		{
+			name:           "all line breaks are needed",
+			message:        "abc\ndef\n\nxyz",
+			autoWrapWidth:  7,
+			expectedResult: "abc\ndef\n\nxyz",
+		},
+		{
+			name:           "some can be unwrapped",
+			message:        "123\nabc def\nghi jkl\nmno\n456\n",
+			autoWrapWidth:  7,
+			expectedResult: "123\nabc def ghi jkl mno\n456\n",
+		},
+	}
+	for _, s := range scenarios {
+		t.Run(s.name, func(t *testing.T) {
+			actualResult := TryRemoveHardLineBreaks(s.message, s.autoWrapWidth)
+			assert.Equal(t, s.expectedResult, actualResult)
+		})
+	}
+}

pkg/gui/controllers/local_commits_controller.go

@@ -354,7 +354,9 @@ func (self *LocalCommitsController) reword(commit *models.Commit) error {
 	if err != nil {
 		return self.c.Error(err)
 	}
-
+	if self.c.UserConfig.Git.Commit.AutoWrapCommitMessage {
+		commitMessage = helpers.TryRemoveHardLineBreaks(commitMessage, self.c.UserConfig.Git.Commit.AutoWrapWidth)
+	}
 	return self.c.Helpers().Commits.OpenCommitMessagePanel(
 		&helpers.OpenCommitMessagePanelOpts{
 			CommitIndex:      self.context().GetSelectedLineIdx(),