terminal: insert mode tests, fix insertBlanks multi-cell char split

src/terminal/Terminal.zig

@@ -1573,6 +1573,11 @@ pub fn insertBlanks(self: *Terminal, count: usize) void {
         // This is the index of the final copyable value that we need to copy.
         const copyable_end = start + copyable - 1;
 
+        // If our last cell we're shifting is wide, then we need to clear
+        // it to be empty so we don't split the multi-cell char.
+        const cell = row.getCellPtr(copyable_end);
+        if (cell.attrs.wide) cell.char = 0;
+
         // Shift count cells. We have to do this backwards since we're not
         // allocated new space, otherwise we'll copy duplicates.
         var i: usize = 0;
@@ -3949,6 +3954,23 @@ test "Terminal: insertBlanks shift off screen" {
     }
 }
 
+test "Terminal: insertBlanks split multi-cell character" {
+    const alloc = testing.allocator;
+    var t = try init(alloc, 5, 10);
+    defer t.deinit(alloc);
+
+    for ("123") |c| try t.print(c);
+    try t.print('橋');
+    t.setCursorPos(1, 1);
+    t.insertBlanks(1);
+
+    {
+        var str = try t.plainString(testing.allocator);
+        defer testing.allocator.free(str);
+        try testing.expectEqualStrings(" 123", str);
+    }
+}
+
 test "Terminal: insertBlanks inside left/right scroll region" {
     const alloc = testing.allocator;
     var t = try init(alloc, 10, 10);
@@ -4073,6 +4095,24 @@ test "Terminal: insert mode with wide characters at end" {
     }
 }
 
+test "Terminal: insert mode pushing off wide character" {
+    const alloc = testing.allocator;
+    var t = try init(alloc, 5, 2);
+    defer t.deinit(alloc);
+
+    for ("123") |c| try t.print(c);
+    try t.print('😀'); // 0x1F600
+    t.modes.set(.insert, true);
+    t.setCursorPos(1, 1);
+    try t.print('X');
+
+    {
+        var str = try t.plainString(testing.allocator);
+        defer testing.allocator.free(str);
+        try testing.expectEqualStrings("X123", str);
+    }
+}
+
 test "Terminal: cursorIsAtPrompt" {
     const alloc = testing.allocator;
     var t = try init(alloc, 3, 2);

website/app/vt/modes/insert/page.mdx

@@ -0,0 +1,89 @@
+import VTMode from "@/components/VTMode";
+
+# Insert
+
+<VTMode value={4} ansi={true} />
+
+When enabled, text is written to the cell under the cursor
+and all existing content is shifted right. When disabled, text
+overwrites existing content.
+
+This mode is unset as part of both [full reset (RIS)](/vt/ris)
+and [soft reset (DECSTR)](/vt/decstr).
+
+If a multi-cell character (such as "橋") is shifted so that the cell is split
+in half, the multi-cell character can either be clipped or erased.
+
+This mode is typically disabled on terminal startup.
+
+## Validation
+
+### INSERT V-1: Simple Usage
+
+```bash
+printf "\033[1;1H" # move to top-left
+printf "\033[0J" # clear screen
+printf "123456"
+printf "\033[1G"
+printf "\033[4h"
+printf "ABC"
+```
+
+```
+|ABC123456_|
+```
+
+### INSERT V-2: Pushing Off the Screen Edge
+
+```bash
+cols=$(tput cols)
+printf "\033[1;1H" # move to top-left
+printf "\033[0J" # clear screen
+printf "\033[${cols}G"
+printf "\033[6D"
+printf "123456"
+printf "\033[6D"
+printf "\033[4h"
+printf "ABC"
+```
+
+```
+|____ABC1234|
+```
+
+### INSERT V-3: Writing on the Screen Edge
+
+```bash
+cols=$(tput cols)
+printf "\033[1;1H" # move to top-left
+printf "\033[0J" # clear screen
+printf "\033[${cols}G"
+printf "\033[6D"
+printf "123456"
+printf "\033[1D"
+printf "\033[4h"
+printf "ABC"
+```
+
+```
+|____12345AB|
+|Cc_________|
+```
+
+### INSERT V-3: Splitting a Multi-Cell Character
+
+```bash
+cols=$(tput cols)
+printf "\033[1;1H" # move to top-left
+printf "\033[0J" # clear screen
+printf "\033[${cols}G"
+printf "\033[6D"
+printf "1234橋"
+printf "\033[6D"
+printf "\033[4h"
+printf "A"
+```
+
+```
+|_____A1234_|
+```

website/app/vt/modes/kam/page.mdx

@@ -13,6 +13,8 @@ A poorly behaved terminal program can lock the terminal emulator
 using this command. Terminal emulators should provide a mechanism
 to reset this or outright disable it.
 
+This mode is typically disabled on terminal startup.
+
 ## Validation
 
 ### KAM V-1: Disable Keyboard Input

website/components/VTMode.tsx

@@ -9,7 +9,7 @@ export default function VTMode({
     <div className="flex my-2.5">
       <div className="border px-1 grid grid-rows-2 grid-cols-1 text-center">
         <div>
-          {ansi ? "?" : ""}
+          {ansi ? "" : "?"}
           {value}
         </div>
       </div>