[FIX] borders: preserve side borders when combining External and All

When applying an External border on a range and then applying All borders on
an inner/overlapping range, the side borders (left/right) of the inner cells
were lost after re-opening the spreadsheet.

The exported border data was correct, but the BordersPlugin recomputation
logic in addBorder incorrectly cleared adjacent borders on import/update. The
adjacency check (adjacent(existingBorder.zone, zone)) reported the side of the
existing zone, but we were deciding whether to clear it based on the same side
key in the new border, instead of the opposite side (shared edge) on the new
zone. As a result, importing the combination of:

C2:C4 with All borders
B2:B4 with left/top/bottom
D2:D4 with right/top/bottom

ended up clearing the left and right borders of C2:C4.

This commit adjusts the adjacent clearing logic to:

Map the adjacent side of the existing zone to the opposite side on the new
zone (i.e. left → right, right → left, top → bottom, bottom → top).

Only clear the existing side if the corresponding opposite side is actually
being written on the new border.

Task: 5270171

Author: dhrp-odoo

src/plugins/core/borders.ts

@@ -342,22 +342,35 @@ export class BordersPlugin extends CorePlugin<BordersPluginState> implements Bor
   ) {
     const borders: ZoneBorder[] = [];
     const plannedBorder = newBorder ? { zone, style: newBorder } : undefined;
-    const sideToClear = {
-      left: force || !!newBorder?.left,
-      right: force || !!newBorder?.right,
-      top: force || !!newBorder?.top,
-      bottom: force || !!newBorder?.bottom,
+
+    // For each side, decide if we must clear the border on the *adjacent*
+    // existing cell when we draw on the opposite side of the new zone.
+    //
+    // Example:
+    //  - newBorder.right is set → we draw border on the RIGHT side of `zone`
+    //  - the cell on the right may already have a LEFT border on that edge
+    // In that case we clear that LEFT border, so only the new RIGHT border
+    // remains on the shared edge.
+    //
+    // existingBorderSideToClear[side] = true means we should clear the border on that
+    // side of the existing adjacent zone before adding the new border.
+    const existingBorderSideToClear = {
+      left: force || !!newBorder?.right,
+      right: force || !!newBorder?.left,
+      top: force || !!newBorder?.bottom,
+      bottom: force || !!newBorder?.top,
     };
     let editingZone: Zone[] = [zone];
     for (const existingBorder of this.borders[sheetId] ?? []) {
       const inter = intersection(existingBorder.zone, zone);
       if (!inter) {
-        // Clear adjacent borders on which you write
+        // Check if the existing border is adjacent to the new zone
         const adjacentEdge = adjacent(existingBorder.zone, zone);
-        if (adjacentEdge && sideToClear[adjacentEdge.position]) {
+        if (adjacentEdge && existingBorderSideToClear[adjacentEdge.position]) {
           for (const newZone of splitIfAdjacent(existingBorder.zone, zone)) {
             const border = this.computeBorderFromZone(newZone, existingBorder);
             const adjacentEdge = adjacent(newZone, zone);
+            // Clear the existing border on the side that touches the new zone
             switch (adjacentEdge?.position) {
               case "left":
                 border.style.left = undefined;

tests/borders/border_plugin.test.ts

@@ -147,6 +147,60 @@ describe("borders", () => {
     });
   });
 
+  test("Preserves side borders when combining external and all via command", () => {
+    const model = new Model();
+    const defaultBorder = DEFAULT_BORDER_DESC;
+
+    setZoneBorders(model, { position: "all" }, ["C3"]);
+    setBordersOnTarget(model, ["C2"], {
+      top: defaultBorder,
+      right: defaultBorder,
+      left: defaultBorder,
+    });
+    setBordersOnTarget(model, ["C4"], {
+      bottom: defaultBorder,
+      left: defaultBorder,
+      right: defaultBorder,
+    });
+    setBordersOnTarget(model, ["B3"], {
+      top: defaultBorder,
+      bottom: defaultBorder,
+      left: defaultBorder,
+    });
+    setBordersOnTarget(model, ["D3"], {
+      top: defaultBorder,
+      bottom: defaultBorder,
+      right: defaultBorder,
+    });
+
+    expect(getBorder(model, "C3")).toEqual({
+      top: defaultBorder,
+      bottom: defaultBorder,
+      left: defaultBorder,
+      right: defaultBorder,
+    });
+    expect(getBorder(model, "C2")).toEqual({
+      top: defaultBorder,
+      left: defaultBorder,
+      right: defaultBorder,
+    });
+    expect(getBorder(model, "C4")).toEqual({
+      bottom: defaultBorder,
+      left: defaultBorder,
+      right: defaultBorder,
+    });
+    expect(getBorder(model, "B3")).toEqual({
+      top: defaultBorder,
+      left: defaultBorder,
+      bottom: defaultBorder,
+    });
+    expect(getBorder(model, "D3")).toEqual({
+      top: defaultBorder,
+      bottom: defaultBorder,
+      right: defaultBorder,
+    });
+  });
+
   test("can set all borders in a zone", () => {
     const model = new Model();