[TCLCORE] The -blockcursor option of text widgets

[Christian W. <Chr...@t-...>]

Dear fellow readers of the core mailing list,

after some playing with the text widget's -blockcursor option my conclusion
is unfortunately, that it is almost unusable when turned on, at least when
the default options of the text widget are in place, which is a very common
use case.

Rationale: the block cursor in its active state (INSERT_ON) draws a rectangle
in the text widget's foreground color. Following that drawing the text is drawn
in the text widget's foreground color resulting in a rectangle in the text
widget's foreground color, i.e.  without any information which was in place
when the widget/cursor was in inactive (INSERT_OFF) state. This makes the
block cursor indefinitely hiding the character under the cursor when the
-insertofftime option is set to zero. Bluntly, the "-blockcursor 1" option
is not usable at all since it's invention, BTW this was in 2003.

Here is a POC how it can be remedied:

---8><---
--- old/tkTextDisp.c
+++ new/tkTextDisp.c
@@ -15,6 +15,7 @@
  
  #include "tkInt.h"
  #include "tkText.h"
+#include "tk3d.h"
  
  #if defined(_WIN32) && !defined(PLATFORM_SDL)
  #include "tkWinInt.h"
@@ -2444,10 +2445,13 @@
  				 * must make sure it's large enough to hold
  				 * line. */
  {
-    TkTextDispChunk *chunkPtr;
+    TkTextDispChunk *chunkPtr, tmpChunk, *otherChunkPtr = NULL;;
      TextDInfo *dInfoPtr = textPtr->dInfoPtr;
      Display *display;
      int height, y_off;
+    struct TextStyle tmpStyle;
+    TkBorder *borderPtr;
+    CharInfo ci;
  #ifndef TK_NO_DOUBLE_BUFFERING
      const int y = 0;
  #else
@@ -2539,6 +2543,38 @@
  	     * here.
  	     */
  
+	    if (textPtr->insertCursorType &&
+		    ((textPtr->flags & (GOT_FOCUS | INSERT_ON)) ==
+			(GOT_FOCUS | INSERT_ON)) &&
+		    (chunkPtr->nextPtr != NULL) &&
+		    (chunkPtr->nextPtr->displayProc == CharDisplayProc) &&
+		    (chunkPtr->nextPtr->numBytes > 0)) {
+		/*
+		 * Make a temporary chunk for displaying the text
+		 * within the block cursor later on.
+		 */
+
+		otherChunkPtr = &tmpChunk;
+		*otherChunkPtr = *(chunkPtr->nextPtr);
+		otherChunkPtr->undisplayProc = NULL;
+		otherChunkPtr->numBytes = 1;
+		tmpStyle = *otherChunkPtr->stylePtr;
+		otherChunkPtr->stylePtr = &tmpStyle;
+		tmpStyle.bgGC = None;
+		borderPtr = (TkBorder *) textPtr->border;
+		tmpStyle.fgGC = borderPtr->bgGC;
+		ci = *((CharInfo *) (otherChunkPtr->clientData));
+		otherChunkPtr->clientData = (ClientData) &ci;
+		ci.numBytes = 1;
+		if ((ci.chars[0] == '\n') ||
+			(ci.chars[0] == ' ') || (ci.chars[0] == '\t')) {
+		    /*
+		     * Ignore newline and other whitespace.
+		     */
+
+		    otherChunkPtr = NULL;
+		}
+	    }
  	    continue;
  	}
  
@@ -2569,6 +2605,28 @@
  		    display, pixmap, dlPtr->y + dlPtr->spaceAbove);
  	}
  
+	if ((otherChunkPtr != NULL) && (textPtr->tkwin != NULL) &&
+		!(textPtr->flags & DESTROYED)) {
+	    /*
+	     * Draw text within the block cursor.
+	     */
+
+	    int x = otherChunkPtr->x + dInfoPtr->x - dInfoPtr->curXPixelOffset;
+
+	    if ((x + otherChunkPtr->width <= 0) || (x >= dInfoPtr->maxX)) {
+		/*
+		 * See note above.
+		 */
+
+		x = -otherChunkPtr->width;
+	    }
+	    otherChunkPtr->displayProc(textPtr, otherChunkPtr, x,
+		    y + dlPtr->spaceAbove, dlPtr->height - dlPtr->spaceAbove -
+		    dlPtr->spaceBelow, dlPtr->baseline - dlPtr->spaceAbove,
+		    display, pixmap, dlPtr->y + dlPtr->spaceAbove);
+	    otherChunkPtr = NULL;
+	}
+
  	if ((textPtr->tkwin == NULL) || (textPtr->flags & DESTROYED)) {
  	    /*
  	     * A displayProc called in the loop above invoked a binding
---8><---

I've tested it in an X11 environment. No idea if this works in Win32 or MacOS.

Cheers,
Christian