Fix offset tracking in insert_newline

#12177 changed `insert_newline`'s behavior to trim any trailing
whitespace on a line which came before a cursor. `insert_newline` would
previously never delete text. Even the whitespace stripping behavior in
#4854 worked by inserting text - a line ending at the beginning of the
line. `global_offs`, a variable that tracks the number of characters
inserted between iterations over the existing selection ranges, was not
updated to also account for text deleted by the trimming behavior,
causing cursors to be offset by the amount of trailing space deleted
and causing panics in some cases.

To fix this we need to subtract the number of trimmed whitespace
characters from `global_offs`. `global_offs` must become an `isize`
(was a `usize`) because it may become negative in cases where a lot of
trailing whitespace is trimmed. Integration tests have been added for
each of these cases.

Fixes #12461
Fixes #12495
Fixes #12539

helix-term/tests/test/commands/insert.rs

@@ -1,5 +1,106 @@
 use super::*;
 
+#[tokio::test(flavor = "multi_thread")]
+async fn insert_newline_many_selections() -> anyhow::Result<()> {
+    test((
+        indoc! {"\
+            #(|o)#ne
+            #(|t)#wo
+            #[|t]#hree
+            "},
+        "i<ret>",
+        indoc! {"\
+            \n#(|o)#ne
+
+            #(|t)#wo
+
+            #[|t]#hree
+            "},
+    ))
+    .await?;
+
+    // In this case the global offset that adjusts selections for inserted and deleted text
+    // should become negative because more text is deleted than is inserted.
+    test((
+        indoc! {"\
+            #[|🏴‍☠️]#          #(|🏴‍☠️)#          #(|🏴‍☠️)#
+            #(|🏴‍☠️)#          #(|🏴‍☠️)#          #(|🏴‍☠️)#
+            "},
+        "i<ret>",
+        indoc! {"\
+            \n#[|🏴‍☠️]#
+            #(|🏴‍☠️)#
+            #(|🏴‍☠️)#
+
+            #(|🏴‍☠️)#
+            #(|🏴‍☠️)#
+            #(|🏴‍☠️)#
+            "},
+    ))
+    .await?;
+
+    // <https://github.com/helix-editor/helix/issues/12495>
+    test((
+        indoc! {"\
+            id #(|1)#,Item #(|1)#,cost #(|1)#,location #(|1)#
+            id #(|2)#,Item #(|2)#,cost #(|2)#,location #(|2)#
+            id #(|1)##(|0)#,Item #(|1)##(|0)#,cost #(|1)##(|0)#,location #(|1)##[|0]#"},
+        "i<ret>",
+        indoc! {"\
+            id
+            #(|1)#,Item
+            #(|1)#,cost
+            #(|1)#,location
+            #(|1)#
+            id
+            #(|2)#,Item
+            #(|2)#,cost
+            #(|2)#,location
+            #(|2)#
+            id
+            #(|1)#
+            #(|0)#,Item
+            #(|1)#
+            #(|0)#,cost
+            #(|1)#
+            #(|0)#,location
+            #(|1)#
+            #[|0]#"},
+    ))
+    .await?;
+
+    // <https://github.com/helix-editor/helix/issues/12461>
+    test((
+        indoc! {"\
+            real R〉 #(||)# 〈real R〉 @ 〈real R〉
+            #(||)# 〈real R〉 + 〈ureal R〉 i #(||)# 〈real R〉 - 〈ureal R〉 i
+            #(||)# 〈real R〉 + i #(||)# 〈real R〉 - i #(||)# 〈real R〉 〈infnan〉 i
+            #(||)# + 〈ureal R〉 i #(||)# - 〈ureal R〉 i
+            #(||)# 〈infnan〉 i #(||)# + i #[||]# - i"},
+        "i<ret>",
+        indoc! {"\
+            real R〉
+            #(||)# 〈real R〉 @ 〈real R〉
+
+            #(||)# 〈real R〉 + 〈ureal R〉 i
+            #(||)# 〈real R〉 - 〈ureal R〉 i
+
+            #(||)# 〈real R〉 + i
+            #(||)# 〈real R〉 - i
+            #(||)# 〈real R〉 〈infnan〉 i
+
+            #(||)# + 〈ureal R〉 i
+            #(||)# - 〈ureal R〉 i
+
+            #(||)# 〈infnan〉 i
+            #(||)# + i
+            #[||]# - i"},
+    ))
+    .await?;
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread")]
 async fn insert_newline_trim_trailing_whitespace() -> anyhow::Result<()> {
     // Trailing whitespace is trimmed.
@@ -117,6 +218,33 @@ async fn insert_newline_continue_line_comment() -> anyhow::Result<()> {
     ))
     .await?;
 
+    // Comment continuation should work on multiple selections.
+    // <https://github.com/helix-editor/helix/issues/12539>
+    test((
+        indoc! {"\
+            ///·Docs#[|·]#
+            pub·struct·A;
+
+            ///·Docs#(|·)#
+            pub·struct·B;
+            "}
+        .replace('·', " "),
+        ":lang rust<ret>i<ret><ret>",
+        indoc! {"\
+            ///·Docs
+            ///
+            ///·#[|·]#
+            pub·struct·A;
+
+            ///·Docs
+            ///
+            ///·#(|·)#
+            pub·struct·B;
+            "}
+        .replace('·', " "),
+    ))
+    .await?;
+
     Ok(())
 }