Left column: fixed width preference, auto scrollbars, pane floors

The tree and palette column took a share of every extra pixel the window
gained. It is now pinned to a width set in Preferences, defaulting to
230 px, so widening the window only ever feeds the wiresheet.

That fixed width brought two existing weaknesses into view:

- Long paths and type names had nowhere to go. Both lists now size their
  tree column to the widest visible row - ttk derives the horizontal
  scroll range from the column width, so a column left stretching can
  never scroll - and carry vertical and horizontal scrollbars that leave
  the grid when they have nothing to scroll.

- A short window squeezed the navigation tree away entirely. Both lists
  asked for ten rows, so the palette requested the most while the tree,
  carrying the larger weight, gave up its height first. The requested
  rows are now 6 and 5, a 300 px minimum window height keeps the
  shortfall shallow, and clamp_left_pane_split holds a dragged sash
  between the two floors.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app_state.py

@@ -19,6 +19,13 @@ DEFAULT_BOX_WIDTH = 100
 MIN_ALLOWED_BOX_WIDTH = 60
 MAX_ALLOWED_BOX_WIDTH = 400
 
+# The tree and palette column is held at this width whatever the window does, so a wider
+# window only ever gives the wiresheet more room. The sash can still be dragged; this is
+# the width the column is set to at startup and whenever the preference changes.
+DEFAULT_LEFT_PANE_WIDTH = 230
+MIN_ALLOWED_LEFT_PANE_WIDTH = 120
+MAX_ALLOWED_LEFT_PANE_WIDTH = 600
+
 # Widget handles, filled in by main once the UI exists
 window = None
 canvas = None
@@ -70,6 +77,7 @@ def load_preferences():
     grid_size = DEFAULT_GRID_SIZE
     add_timestamp = DEFAULT_ADD_TIMESTAMP
     box_width = DEFAULT_BOX_WIDTH
+    left_pane_width = DEFAULT_LEFT_PANE_WIDTH
     sedona_home = ""
 
     if os.path.exists(PROPERTIES_FILE):
@@ -78,6 +86,9 @@ def load_preferences():
             grid_size = config.getint("Editor", "grid_size", fallback=DEFAULT_GRID_SIZE)
             add_timestamp = config.getboolean("Editor", "add_timestamp", fallback=DEFAULT_ADD_TIMESTAMP)
             box_width = config.getint("Editor", "box_width", fallback=DEFAULT_BOX_WIDTH)
+            left_pane_width = config.getint(
+                "Editor", "left_pane_width", fallback=DEFAULT_LEFT_PANE_WIDTH
+            )
             sedona_home = config.get("Editor", "sedona.home", fallback="").strip()
         except Exception:
             pass
@@ -86,7 +97,8 @@ def load_preferences():
             "sedona.home": "",
             "grid_size": str(DEFAULT_GRID_SIZE),
             "add_timestamp": str(DEFAULT_ADD_TIMESTAMP),
-            "box_width": str(DEFAULT_BOX_WIDTH)
+            "box_width": str(DEFAULT_BOX_WIDTH),
+            "left_pane_width": str(DEFAULT_LEFT_PANE_WIDTH)
         }
         try:
             with open(PROPERTIES_FILE, "w") as configfile:
@@ -95,7 +107,10 @@ def load_preferences():
             pass
 
     box_width = max(MIN_ALLOWED_BOX_WIDTH, min(MAX_ALLOWED_BOX_WIDTH, box_width))
-    return sedona_home, grid_size, add_timestamp, box_width
+    left_pane_width = max(
+        MIN_ALLOWED_LEFT_PANE_WIDTH, min(MAX_ALLOWED_LEFT_PANE_WIDTH, left_pane_width)
+    )
+    return sedona_home, grid_size, add_timestamp, box_width, left_pane_width
 
 
 def save_preferences():
@@ -105,7 +120,8 @@ def save_preferences():
         "sedona.home": SEDONA_HOME,
         "grid_size": str(GRID_SIZE),
         "add_timestamp": str(ADD_TIMESTAMP),
-        "box_width": str(BOX_WIDTH)
+        "box_width": str(BOX_WIDTH),
+        "left_pane_width": str(LEFT_PANE_WIDTH)
     }
     with open(PROPERTIES_FILE, "w") as configfile:
         config.write(configfile)
@@ -148,4 +164,4 @@ def mark_clean():
         canvas.config(bg="white")
 
 
-SEDONA_HOME, GRID_SIZE, ADD_TIMESTAMP, BOX_WIDTH = load_preferences()
+SEDONA_HOME, GRID_SIZE, ADD_TIMESTAMP, BOX_WIDTH, LEFT_PANE_WIDTH = load_preferences()