remove obsolete full-row-repaint debug toggle; document diagnostics

The fullRowRepaint toggle existed only to bisect the black-bar repaint artifact,
which is now fixed, so drop it. Document the two remaining render-debug flags
(profile, debugRepaint) in the README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -14,3 +14,4 @@ build
 build
 .gradle
 bin
+.worktrees

README.md

@@ -132,3 +132,22 @@ open_scrollback = "ALT+S"
 Each tab has its own stack of tiled and floating panes; only the active tab is rendered. A
 thin tab bar appears at the top when more than one tab is open. Closing the last tiled pane
 while floating panes exist promotes the most recently active floating pane to a tiled pane.
+
+## Diagnostics
+
+Two render-debugging flags are off by default and add no overhead unless enabled. Pass them
+as JVM system properties (each also has an environment-variable equivalent). With the
+packaged binary the JVM picks them up from `JDK_JAVA_OPTIONS`:
+
+```sh
+JDK_JAVA_OPTIONS="-Djprototerm.profile=true" ./result/bin/jprototerm
+```
+
+- `-Djprototerm.profile=true` (or `JPROTOTERM_PROFILE=1`): print a `[render-profile]` line to
+  stderr every N renders with the per-frame cost of each render stage — `snapshot` (terminal
+  state marshalling), `fingerprint` (change detection), `draw` (canvas painting), and the
+  `frame-total`. Set `-Djprototerm.profile.frames=<N>` to change the dump interval (default
+  120).
+- `-Djprototerm.debugRepaint=true` (or `JPROTOTERM_DEBUG_REPAINT=1`): paint each per-column
+  repaint run's cleared span red instead of clearing it. Repainted regions flash red, so you
+  can see exactly which cells are being redrawn each frame.

flake.lock

@@ -70,11 +70,11 @@
         "nixpkgs": "nixpkgs"
       },
       "locked": {
-        "lastModified": 1780256181,
-        "narHash": "sha256-/saXdnYMbAMfP7u6USSqtNkBIgqZhU+CPr3F8tUQhHU=",
+        "lastModified": 1780258814,
+        "narHash": "sha256-8rxL7xaZ/loYg3zdt0w5+hfNyHFVknDZN360NzrtCsQ=",
         "ref": "refs/heads/main",
-        "rev": "db5ee5d20daf8855de3a3b2fa9349eced70946f0",
-        "revCount": 21,
+        "rev": "6a3d5aa0b0b1f738c958e2a2f0249574c07d9c4d",
+        "revCount": 23,
         "type": "git",
         "url": "https://gitea.gregorlohaus.com/gregor/jlibghostty.git"
       },

src/main/java/com/gregor/jprototerm/KeyEncoder.java

@@ -26,7 +26,7 @@ final class KeyEncoder {
         return switch (code) {
             case ENTER -> "\r";
             case BACK_SPACE -> "\u007f";
-            case TAB -> "\t";
+            case TAB -> event.isShiftDown() ? "\u001b[Z" : "\t";
             case ESCAPE -> "\u001b";
             case UP -> "\u001b[A";
             case DOWN -> "\u001b[B";

src/main/java/com/gregor/jprototerm/TerminalPaneNode.java

@@ -44,11 +44,12 @@ final class TerminalPaneNode extends Region {
     private static final int DIRTY_PARTIAL = 1;
     private static final int DIRTY_FULL = 2;
 
-    // Debug toggle: when set, skip the per-column repaint and always repaint the whole row.
-    // Used to bisect partial-repaint artifacts (stale black bars near the cursor).
-    private static final boolean FULL_ROW_REPAINT =
-            Boolean.getBoolean("jprototerm.fullRowRepaint")
-                    || "1".equals(System.getenv("JPROTOTERM_FULL_ROW_REPAINT"));
+    // Debug toggle: paint each repaint run's cleared span red instead of clearing it to
+    // transparent. If the black bars turn red, they are spans repaintColumns clears but never
+    // refills; if they stay black, those pixels are never touched by repaintColumns at all.
+    private static final boolean DEBUG_REPAINT =
+            Boolean.getBoolean("jprototerm.debugRepaint")
+                    || "1".equals(System.getenv("JPROTOTERM_DEBUG_REPAINT"));
 
     private static final Color DEFAULT_FOREGROUND = Color.rgb(225, 229, 235);
     private static final Color SELECTED_BACKGROUND = Color.rgb(52, 92, 140);
@@ -740,10 +741,6 @@ final class TerminalPaneNode extends Region {
         }
 
         private void renderChanged(RenderRow row) {
-            if (FULL_ROW_REPAINT) {
-                render(row);
-                return;
-            }
             double oldWidth = canvas.getWidth();
             double oldHeight = canvas.getHeight();
             prepareCanvas(row);
@@ -839,7 +836,15 @@ final class TerminalPaneNode extends Region {
             double x = TerminalMetrics.PADDING + startColumn * cellWidth;
             double width = (endColumn - startColumn + 1) * cellWidth;
 
-            gc.clearRect(x, 0.0, width, canvas.getHeight());
+            // Opaque base fill rather than clearRect: a transparent clear leaves the run's
+            // fractional edge pixels transparent, which show the near-black pane background
+            // as a thin seam bar against the adjacent (un-repainted) line. Filling opaque
+            // removes every transparent pixel; per-cell backgrounds then paint on top, and
+            // default-background cells correctly show PANE_BACKGROUND. Safe because the
+            // per-column path never runs while kitty graphics (which need a transparent row
+            // canvas for below-text images) are present.
+            gc.setFill(DEBUG_REPAINT ? Color.RED : PANE_BACKGROUND);
+            gc.fillRect(x, 0.0, width, canvas.getHeight());
             if (startColumn == 0) {
                 gc.setFill(rowEdgeBackground(row, true));
                 gc.fillRect(0.0, 0.0, TerminalMetrics.PADDING, canvas.getHeight());