Calculate and save the correct HID report size per device

src/shims/node-hid.ts

@@ -177,9 +177,29 @@ const ExtendedHID = {
       const data = new Uint8Array(arr.slice(1));
       await this._hidDevice?._device.sendReport(0, data);
     }
+
+    async getReportSize(): Promise<number> {
+      const device = this._hidDevice?._device;
+
+      // Look explicitely for a VIA collection. There may be others. 
+      const viaCollection = device?.collections
+        .find(c => c.usagePage === 0xFF60 && c.usage === 0x61);
+
+      // Sum up lengths of all the fields of the to get the size of output report item
+      const bits = (viaCollection?.outputReports?.[0]?.items ?? []).reduce(
+        (sum: number, item: HIDReportItem) =>
+          sum + (item.reportSize ?? 0) * (item.reportCount ?? 0),
+        0
+      );
+
+      return bits > 0 ? Math.ceil(bits / 8) : 32; // fallback for standard VIA keyboards
+    }
+
   },
 };
 
+
+
 const promisify = (cb: Function) => () => {
   return new Promise((res, rej) => {
     cb((e: any, d: any) => {

src/utils/keyboard-api.ts

@@ -145,6 +145,7 @@ export const canConnect = (device: Device) => {
 
 export class KeyboardAPI {
   kbAddr: HIDAddress;
+  private _reportSize?: number;
 
   constructor(path: string) {
     this.kbAddr = path;
@@ -642,9 +643,20 @@ export class KeyboardAPI {
     return cache[this.kbAddr].hid;
   }
 
+  async getReportSize(): Promise<number> {
+    if (this._reportSize !== undefined) {
+      return this._reportSize;
+    }
+    let size = await this.getHID().getReportSize();
+
+    this._reportSize = size;
+    return size;
+  }
+
   async _hidCommand(command: Command, bytes: Array<number> = []): Promise<any> {
     const commandBytes = [...[COMMAND_START, command], ...bytes];
-    const paddedArray = new Array(33).fill(0);
+    const arraySize = await this.getReportSize() + 1;
+    const paddedArray = new Array(arraySize).fill(0);
     commandBytes.forEach((val, idx) => {
       paddedArray[idx] = val;
     });