Description Mike Parker 2011-08-05 11:30:12 EDT

Message dialogs use labels with the SWT.WRAP style to display the message, which causes the label to be created with the SS_EDITCONTROL style flag:

Label.java (#widgetStyle()):

	if (OS.WIN32_VERSION >= OS.VERSION (5, 0)) {
		if ((style & SWT.WRAP) != 0) bits |= OS.SS_EDITCONTROL;
	} 

I wanted to format Message dialog text into columns using tab characters (\t), using GC#textExtent() to figure out how many tab characters to insert to reach a particular column.

This approach failed, however, because it proved to be impossible to correctly work out the extent of a string containing one or more tab characters.

Reason: The SS_EDITCONTROL style flag alters the tab spacings! For example, for 8pt Tahoma (the default), the tab spacing within a label widget was 48 with SS_EDITCONTROL set, but only 40 without. To obtain the correct text extents in the former case, it would be necessary for GC#textExtent() to call DrawText() with the DT_EDITCONTROL flag set, which is unfortunately not one of the supported DT_* flags (see snippet below):

GC.java (#textExtent(String, int)):

	int uFormat = OS.DT_LEFT | OS.DT_CALCRECT;
	if ((flags & SWT.DRAW_DELIMITER) == 0) uFormat |= OS.DT_SINGLELINE;
	if ((flags & SWT.DRAW_TAB) != 0) uFormat |= OS.DT_EXPANDTABS;
	if ((flags & SWT.DRAW_MNEMONIC) == 0) uFormat |= OS.DT_NOPREFIX;
	OS.DrawText(handle, buffer, buffer.length(), rect, uFormat);

Analogously to the code in Label.java (see above), I was expecting to be able to indirectly set the DT_EDITCONTROL flag by calling #textExtent() with the SWT.WRAP flag:

	if ((flags & SWT.WRAP) != 0) uFormat |= OS.DT_EDITCONTROL;

Could this line be added in to the source code base?