Fix cursor artifacts and WM_TIMER starvation during high-speed data

Three fixes:

1. Cursor font mismatch: cursor overlay TextOut used the default DC
   font instead of FPaintFont (OEM_CHARSET). The wrong font size left
   residual pixels that buffer BitBlt didn't fully cover. Fix: select
   FPaintFont + OPAQUE BkMode into the CS_OWNDC screen DC once in
   RecalcCellSize, and re-assert before cursor TextOut in case VCL
   Paint altered the DC state.

2. WM_TIMER starvation: on Win16, WM_TIMER is lowest-priority and
   never dispatched while WM_COMMNOTIFY floods the queue. Fix: render
   from EndUpdate using GetTickCount throttle (no timer dependency).
   The timer remains as fallback for idle blink and trailing data.

3. Scroll performance: restore deferred ScrollDC at render time (not
   during parsing) so only newly-revealed rows need PaintLine instead
   of all 25. Dirty flags shift with each DoScrollUp to track the
   correct logical line positions after scroll.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

delphi/KPCOMM.PAS

@@ -351,7 +351,6 @@ end;
 
 function TKPComm.GetInput: string;
 var
-  Stat:        TComStat;
   BytesToRead: Integer;
   BytesRead:   Integer;
   Buf:         array[0..255] of Char;
@@ -360,15 +359,12 @@ begin
   if not FPortOpen or (FCommId < 0) then
     Exit;
 
-  GetCommError(FCommId, Stat);
+  { Read directly without querying GetCommError first.  ReadComm      }
-  BytesToRead := Stat.cbInQue;
+  { returns the number of bytes actually available (up to BytesToRead) }
-
+  { so the extra GetCommError round-trip is unnecessary overhead.      }
+  BytesToRead := 255;
   if (FInputLen > 0) and (BytesToRead > FInputLen) then
     BytesToRead := FInputLen;
-  if BytesToRead > 255 then
-    BytesToRead := 255;
-  if BytesToRead <= 0 then
-    Exit;
 
   BytesRead := ReadComm(FCommId, @Buf, BytesToRead);
   if BytesRead <= 0 then
@@ -558,13 +554,11 @@ end;
 
 
 procedure TKPComm.ProcessReceiveNotify;
-var
-  Stat: TComStat;
 begin
   if FRThreshold <= 0 then
     Exit;
-  GetCommError(FCommId, Stat);
+  { WM_COMMNOTIFY with CN_RECEIVE means data is available -- the driver }
-  if Integer(Stat.cbInQue) >= FRThreshold then
+  { already checked the threshold.  No need to call GetCommError here.  }
   DoCommEvent(comEvReceive);
 end;
 

delphi/README.md

@@ -116,6 +116,9 @@ CSI (ESC\[) sequences:
 | ESC\[*params*m | SGR | Set graphic rendition (see below) |
 | ESC\[s | SCP | Save cursor position |
 | ESC\[u | RCP | Restore cursor position |
+| ESC\[c | DA | Device Attributes — responds ESC\[?1;0c (VT100) |
+| ESC\[5n | DSR | Device Status Report — responds ESC\[0n (OK) |
+| ESC\[6n | CPR | Cursor Position Report — responds ESC\[*row*;*col*R |
 
 SGR codes:
 
@@ -148,6 +151,7 @@ Control characters:
 | LF (#10) | Line feed (scrolls at bottom) |
 | BS (#8) | Backspace (no erase) |
 | TAB (#9) | Tab to next 8-column stop |
+| ENQ (#5) | Answerback — responds ESC\[?1;0c (VT100) |
 | BEL (#7) | System beep |
 
 **ANSI Music:**

delphi/TESTMAIN.PAS

@@ -85,9 +85,19 @@ begin
   case FComm.CommEvent of
     comEvReceive:
       begin
+        { Drain all available data in a single update batch.  This    }
+        { suppresses per-Write rendering so we get one paint at the   }
+        { end instead of one per 255-byte chunk.                      }
+        FAnsi.BeginUpdate;
+        try
+          repeat
             S := FComm.Input;
             if Length(S) > 0 then
               FAnsi.Write(S);
+          until Length(S) = 0;
+        finally
+          FAnsi.EndUpdate;
+        end;
       end;
   end;
 end;
@@ -131,7 +141,7 @@ begin
   FEditSettings.Left   := 148;
   FEditSettings.Top    := 8;
   FEditSettings.Width  := 140;
-  FEditSettings.Text   := '9600,N,8,1';
+  FEditSettings.Text   := '115200,N,8,1';
 
   FBtnOpen := TButton.Create(Self);
   FBtnOpen.Parent  := Self;
@@ -164,6 +174,15 @@ begin
   FAnsi.Left      := 0;
   FAnsi.Top       := 38;
   FAnsi.OnKeyData := AnsiKeyData;
+
+  { Font diagnostic: write known CP437 box-drawing characters.         }
+  { If the OEM font is working, you should see:                        }
+  {   Line 1: single-line box top    ┌───┐                             }
+  {   Line 2: shade + full block     ░▒▓█                              }
+  {   Line 3: single-line box bottom └───┘                             }
+  { If you see accented letters (Ú Ä ¿ ° ± ² Û À Ù), the font is     }
+  { ANSI_CHARSET instead of OEM_CHARSET.                               }
+  FAnsi.Write(#$DA#$C4#$C4#$C4#$BF' '#$B0#$B1#$B2#$DB' '#$C0#$C4#$C4#$C4#$D9#13#10);
 end;
 
 

drv/commdrv.c

@@ -259,6 +259,13 @@ void applyBaudRate(PortStateT *port, uint16_t baud)
     base    = port->baseAddr;
     divisor = (uint16_t)(BAUD_DIVISOR_BASE / actualBaud);
 
+    // Guard: divisor 0 means the UART treats it as 65536, giving ~1.76 baud.
+    // This can happen when BuildCommDCB stores a raw truncated value for
+    // 115200 (e.g. 0xE101 = 57601) and a future rate exceeds 115200.
+    if (divisor == 0) {
+        divisor = 1;
+    }
+
     // Set DLAB to access divisor latch
     lcr = (uint8_t)_inp(base + UART_LCR);
     _outp(base + UART_LCR, lcr | LCR_DLAB);
@@ -315,6 +322,11 @@ void applyLineParams(PortStateT *port, uint8_t byteSize, uint8_t parity, uint8_t
             break;
     }
 
+    dbgHex16("KPCOMM: applyLine byteSize", (uint16_t)byteSize);
+    dbgHex16("KPCOMM: applyLine parity", (uint16_t)parity);
+    dbgHex16("KPCOMM: applyLine stopBits", (uint16_t)stopBits);
+    dbgHex16("KPCOMM: applyLine LCR", (uint16_t)lcr);
+
     _outp(base + UART_LCR, lcr);
 
     port->byteSize = byteSize;
@@ -952,6 +964,8 @@ int16_t FAR PASCAL _export inicom(DCB FAR *dcb)
     port->comDeb.baudRate   = port->baudRate;
     port->comDeb.qInSize    = port->rxSize;
     port->comDeb.qOutSize   = port->txSize;
+    port->comDeb.lcrShadow  = (uint8_t)_inp(port->baseAddr + UART_LCR);
+    port->comDeb.mcrShadow  = (uint8_t)_inp(port->baseAddr + UART_MCR);
 
     // Enable receive and line status interrupts
     _outp(port->baseAddr + UART_IER, IER_RDA | IER_LSI | IER_MSI);
@@ -1101,7 +1115,9 @@ void primeTx(PortStateT *port)
 // reactivateOpenCommPorts - Reactivate all ports after task switch (ordinal 18)
 //
 // Called by Windows when switching back to this VM.
-// Re-enables interrupts and restores MCR state.
+// Restores full UART state: baud rate, line params (LCR), MCR, FIFOs,
+// and re-enables interrupts.  A DOS fullscreen app or VM switch may
+// have reprogrammed the UART, so we must restore everything.
 // -----------------------------------------------------------------------
 void FAR PASCAL _export reactivateOpenCommPorts(void)
 {
@@ -1117,6 +1133,12 @@ void FAR PASCAL _export reactivateOpenCommPorts(void)
             continue;
         }
 
+        // Restore baud rate (sets DLAB, writes divisor, clears DLAB)
+        applyBaudRate(port, port->baudRate);
+
+        // Restore line parameters (word length, parity, stop bits)
+        applyLineParams(port, port->byteSize, port->parity, port->stopBits);
+
         // Restore MCR (DTR, RTS, OUT2)
         mcr = MCR_OUT2;
         if (port->dtrState) {